Module:Date

    From Nonbinary Wiki
    Revision as of 02:55, 3 August 2020 by wikipedia>Johnuniq (update from sandbox: implement show=M (minutes) and show=s (seconds); better method to fill a partial date)

    Documentation for this module may be created at Module:Date/doc

    -- Date functions for use by other modules.
    -- I18N and time zones are not supported.
    
    local MINUS = '−'  -- Unicode U+2212 MINUS SIGN
    local floor = math.floor
    
    local Date, DateDiff, diffmt  -- forward declarations
    local uniq = { 'unique identifier' }
    
    local function is_date(t)
    	-- The system used to make a date read-only means there is no unique
    	-- metatable that is conveniently accessible to check.
    	return type(t) == 'table' and t._id == uniq
    end
    
    local function is_diff(t)
    	return type(t) == 'table' and getmetatable(t) == diffmt
    end
    
    local function _list_join(list, sep)
    	return table.concat(list, sep)
    end
    
    local function collection()
    	-- Return a table to hold items.
    	return {
    		n = 0,
    		add = function (self, item)
    			self.n = self.n + 1
    			self[self.n] = item
    		end,
    		join = _list_join,
    	}
    end
    
    local function strip_to_nil(text)
    	-- If text is a string, return its trimmed content, or nil if empty.
    	-- Otherwise return text (convenient when Date fields are provided from
    	-- another module which may pass a string, a number, or another type).
    	if type(text) == 'string' then
    		text = text:match('(%S.-)%s*$')
    	end
    	return text
    end
    
    local function is_leap_year(year, calname)
    	-- Return true if year is a leap year.
    	if calname == 'Julian' then
    		return year % 4 == 0
    	end
    	return (year % 4 == 0 and year % 100 ~= 0) or year % 400 == 0
    end
    
    local function days_in_month(year, month, calname)
    	-- Return number of days (1..31) in given month (1..12).
    	if month == 2 and is_leap_year(year, calname) then
    		return 29
    	end
    	return ({ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 })[month]
    end
    
    local function h_m_s(time)
    	-- Return hour, minute, second extracted from fraction of a day.
    	time = floor(time * 24 * 3600 + 0.5)  -- number of seconds
    	local second = time % 60
    	time = floor(time / 60)
    	return floor(time / 60), time % 60, second
    end
    
    local function hms(date)
    	-- Return fraction of a day from date's time, where (0 <= fraction < 1)
    	-- if the values are valid, but could be anything if outside range.
    	return (date.hour + (date.minute + date.second / 60) / 60) / 24
    end
    
    local function julian_date(date)
    	-- Return jd, jdz from a Julian or Gregorian calendar date where
    	--   jd = Julian date and its fractional part is zero at noon
    	--   jdz = same, but assume time is 00:00:00 if no time given
    	-- http://www.tondering.dk/claus/cal/julperiod.php#formula
    	-- Testing shows this works for all dates from year -9999 to 9999!
    	-- JDN 0 is the 24-hour period starting at noon UTC on Monday
    	--    1 January 4713 BC  = (-4712, 1, 1)   Julian calendar
    	--   24 November 4714 BC = (-4713, 11, 24) Gregorian calendar
    	local offset
    	local a = floor((14 - date.month)/12)
    	local y = date.year + 4800 - a
    	if date.calendar == 'Julian' then
    		offset = floor(y/4) - 32083
    	else
    		offset = floor(y/4) - floor(y/100) + floor(y/400) - 32045
    	end
    	local m = date.month + 12*a - 3
    	local jd = date.day + floor((153*m + 2)/5) + 365*y + offset
    	if date.hastime then
    		jd = jd + hms(date) - 0.5
    		return jd, jd
    	end
    	return jd, jd - 0.5
    end
    
    local function set_date_from_jd(date)
    	-- Set the fields of table date from its Julian date field.
    	-- Return true if date is valid.
    	-- http://www.tondering.dk/claus/cal/julperiod.php#formula
    	-- This handles the proleptic Julian and Gregorian calendars.
    	-- Negative Julian dates are not defined but they work.
    	local calname = date.calendar
    	local low, high  -- min/max limits for date ranges −9999-01-01 to 9999-12-31
    	if calname == 'Gregorian' then
    		low, high = -1930999.5, 5373484.49999
    	elseif calname == 'Julian' then
    		low, high = -1931076.5, 5373557.49999
    	else
    		return
    	end
    	local jd = date.jd
    	if not (type(jd) == 'number' and low <= jd and jd <= high) then
    		return
    	end
    	local jdn = floor(jd)
    	if date.hastime then
    		local time = jd - jdn  -- 0 <= time < 1
    		if time >= 0.5 then    -- if at or after midnight of next day
    			jdn = jdn + 1
    			time = time - 0.5
    		else
    			time = time + 0.5
    		end
    		date.hour, date.minute, date.second = h_m_s(time)
    	else
    		date.second = 0
    		date.minute = 0
    		date.hour = 0
    	end
    	local b, c
    	if calname == 'Julian' then
    		b = 0
    		c = jdn + 32082
    	else  -- Gregorian
    		local a = jdn + 32044
    		b = floor((4*a + 3)/146097)
    		c = a - floor(146097*b/4)
    	end
    	local d = floor((4*c + 3)/1461)
    	local e = c - floor(1461*d/4)
    	local m = floor((5*e + 2)/153)
    	date.day = e - floor((153*m + 2)/5) + 1
    	date.month = m + 3 - 12*floor(m/10)
    	date.year = 100*b + d - 4800 + floor(m/10)
    	return true
    end
    
    local function fix_numbers(numbers, y, m, d, H, M, S, partial, hastime, calendar)
    	-- Put the result of normalizing the given values in table numbers.
    	-- The result will have valid m, d values if y is valid; caller checks y.
    	-- The logic of PHP mktime is followed where m or d can be zero to mean
    	-- the previous unit, and -1 is the one before that, etc.
    	-- Positive values carry forward.
    	local date
    	if not (1 <= m and m <= 12) then
    		date = Date(y, 1, 1)
    		if not date then return end
    		date = date + ((m - 1) .. 'm')
    		y, m = date.year, date.month
    	end
    	local days_hms
    	if not partial then
    		if hastime and H and M and S then
    			if not (0 <= H and H <= 23 and
    					0 <= M and M <= 59 and
    					0 <= S and S <= 59) then
    				days_hms = hms({ hour = H, minute = M, second = S })
    			end
    		end
    		if days_hms or not (1 <= d and d <= days_in_month(y, m, calendar)) then
    			date = date or Date(y, m, 1)
    			if not date then return end
    			date = date + (d - 1 + (days_hms or 0))
    			y, m, d = date.year, date.month, date.day
    			if days_hms then
    				H, M, S = date.hour, date.minute, date.second
    			end
    		end
    	end
    	numbers.year = y
    	numbers.month = m
    	numbers.day = d
    	if days_hms then
    		-- Don't set H unless it was valid because a valid H will set hastime.
    		numbers.hour = H
    		numbers.minute = M
    		numbers.second = S
    	end
    end
    
    local function set_date_from_numbers(date, numbers, options)
    	-- Set the fields of table date from numeric values.
    	-- Return true if date is valid.
    	if type(numbers) ~= 'table' then
    		return
    	end
    	local y = numbers.year   or date.year
    	local m = numbers.month  or date.month
    	local d = numbers.day    or date.day
    	local H = numbers.hour
    	local M = numbers.minute or date.minute or 0
    	local S = numbers.second or date.second or 0
    	local need_fix
    	if y and m and d then
    		date.partial = nil
    		if not (-9999 <= y and y <= 9999 and
    			1 <= m and m <= 12 and
    			1 <= d and d <= days_in_month(y, m, date.calendar)) then
    				if not date.want_fix then
    					return
    				end
    				need_fix = true
    		end
    	elseif y and date.partial then
    		if d or not (-9999 <= y and y <= 9999) then
    			return
    		end
    		if m and not (1 <= m and m <= 12) then
    			if not date.want_fix then
    				return
    			end
    			need_fix = true
    		end
    	else
    		return
    	end
    	if date.partial then
    		H = nil  -- ignore any time
    		M = nil
    		S = nil
    	else
    		if H then
    			-- It is not possible to set M or S without also setting H.
    			date.hastime = true
    		else
    			H = 0
    		end
    		if not (0 <= H and H <= 23 and
    				0 <= M and M <= 59 and
    				0 <= S and S <= 59) then
    			if date.want_fix then
    				need_fix = true
    			else
    				return
    			end
    		end
    	end
    	date.want_fix = nil
    	if need_fix then
    		fix_numbers(numbers, y, m, d, H, M, S, date.partial, date.hastime, date.calendar)
    		return set_date_from_numbers(date, numbers, options)
    	end
    	date.year = y    -- -9999 to 9999 ('n BC' → year = 1 - n)
    	date.month = m   -- 1 to 12 (may be nil if partial)
    	date.day = d     -- 1 to 31 (* = nil if partial)
    	date.hour = H    -- 0 to 59 (*)
    	date.minute = M  -- 0 to 59 (*)
    	date.second = S  -- 0 to 59 (*)
    	if type(options) == 'table' then
    		for _, k in ipairs({ 'am', 'era', 'format' }) do
    			if options[k] then
    				date.options[k] = options[k]
    			end
    		end
    	end
    	return true
    end
    
    local function make_option_table(options1, options2)
    	-- If options1 is a string, return a table with its settings, or
    	-- if it is a table, use its settings.
    	-- Missing options are set from table options2 or defaults.
    	-- If a default is used, a flag is set so caller knows the value was not intentionally set.
    	-- Valid option settings are:
    	-- am: 'am', 'a.m.', 'AM', 'A.M.'
    	--     'pm', 'p.m.', 'PM', 'P.M.' (each has same meaning as corresponding item above)
    	-- era: 'BCMINUS', 'BCNEGATIVE', 'BC', 'B.C.', 'BCE', 'B.C.E.', 'AD', 'A.D.', 'CE', 'C.E.'
    	-- Option am = 'am' does not mean the hour is AM; it means 'am' or 'pm' is used, depending on the hour,
    	--    and am = 'pm' has the same meaning.
    	-- Similarly, era = 'BC' means 'BC' is used if year <= 0.
    	-- BCMINUS displays a MINUS if year < 0 and the display format does not include %{era}.
    	-- BCNEGATIVE is similar but displays a hyphen.
    	local result = { bydefault = {} }
    	if type(options1) == 'table' then
    		result.am = options1.am
    		result.era = options1.era
    	elseif type(options1) == 'string' then
    		-- Example: 'am:AM era:BC' or 'am=AM era=BC'.
    		for item in options1:gmatch('%S+') do
    			local lhs, rhs = item:match('^(%w+)[:=](.+)$')
    			if lhs then
    				result[lhs] = rhs
    			end
    		end
    	end
    	options2 = type(options2) == 'table' and options2 or {}
    	local defaults = { am = 'am', era = 'BC' }
    	for k, v in pairs(defaults) do
    		if not result[k] then
    			if options2[k] then
    				result[k] = options2[k]
    			else
    				result[k] = v
    				result.bydefault[k] = true
    			end
    		end
    	end
    	return result
    end
    
    local ampm_options = {
    	-- lhs = input text accepted as an am/pm option
    	-- rhs = code used internally
    	['am']   = 'am',
    	['AM']   = 'AM',
    	['a.m.'] = 'a.m.',
    	['A.M.'] = 'A.M.',
    	['pm']   = 'am',  -- same as am
    	['PM']   = 'AM',
    	['p.m.'] = 'a.m.',
    	['P.M.'] = 'A.M.',
    }
    
    local era_text = {
    	-- Text for displaying an era with a positive year (after adjusting
    	-- by replacing year with 1 - year if date.year <= 0).
    	-- options.era = { year<=0 , year>0 }
    	['BCMINUS']    = { 'BC'    , ''    , isbc = true, sign = MINUS },
    	['BCNEGATIVE'] = { 'BC'    , ''    , isbc = true, sign = '-'   },
    	['BC']         = { 'BC'    , ''    , isbc = true },
    	['B.C.']       = { 'B.C.'  , ''    , isbc = true },
    	['BCE']        = { 'BCE'   , ''    , isbc = true },
    	['B.C.E.']     = { 'B.C.E.', ''    , isbc = true },
    	['AD']         = { 'BC'    , 'AD'   },
    	['A.D.']       = { 'B.C.'  , 'A.D.' },
    	['CE']         = { 'BCE'   , 'CE'   },
    	['C.E.']       = { 'B.C.E.', 'C.E.' },
    }
    
    local function get_era_for_year(era, year)
    	return (era_text[era] or era_text['BC'])[year > 0 and 2 or 1] or ''
    end
    
    local function strftime(date, format, options)
    	-- Return date formatted as a string using codes similar to those
    	-- in the C strftime library function.
    	local sformat = string.format
    	local shortcuts = {
    		['%c'] = '%-I:%M %p %-d %B %-Y %{era}',  -- date and time: 2:30 pm 1 April 2016
    		['%x'] = '%-d %B %-Y %{era}',            -- date:          1 April 2016
    		['%X'] = '%-I:%M %p',                    -- time:          2:30 pm
    	}
    	if shortcuts[format] then
    		format = shortcuts[format]
    	end
    	local codes = {
    		a = { field = 'dayabbr' },
    		A = { field = 'dayname' },
    		b = { field = 'monthabbr' },
    		B = { field = 'monthname' },
    		u = { fmt = '%d'  , field = 'dowiso' },
    		w = { fmt = '%d'  , field = 'dow' },
    		d = { fmt = '%02d', fmt2 = '%d', field = 'day' },
    		m = { fmt = '%02d', fmt2 = '%d', field = 'month' },
    		Y = { fmt = '%04d', fmt2 = '%d', field = 'year' },
    		H = { fmt = '%02d', fmt2 = '%d', field = 'hour' },
    		M = { fmt = '%02d', fmt2 = '%d', field = 'minute' },
    		S = { fmt = '%02d', fmt2 = '%d', field = 'second' },
    		j = { fmt = '%03d', fmt2 = '%d', field = 'dayofyear' },
    		I = { fmt = '%02d', fmt2 = '%d', field = 'hour', special = 'hour12' },
    		p = { field = 'hour', special = 'am' },
    	}
    	options = make_option_table(options, date.options)
    	local amopt = options.am
    	local eraopt = options.era
    	local function replace_code(spaces, modifier, id)
    		local code = codes[id]
    		if code then
    			local fmt = code.fmt
    			if modifier == '-' and code.fmt2 then
    				fmt = code.fmt2
    			end
    			local value = date[code.field]
    			if not value then
    				return nil  -- an undefined field in a partial date
    			end
    			local special = code.special
    			if special then
    				if special == 'hour12' then
    					value = value % 12
    					value = value == 0 and 12 or value
    				elseif special == 'am' then
    					local ap = ({
    						['a.m.'] = { 'a.m.', 'p.m.' },
    						['AM'] = { 'AM', 'PM' },
    						['A.M.'] = { 'A.M.', 'P.M.' },
    					})[ampm_options[amopt]] or { 'am', 'pm' }
    					return (spaces == '' and '' or '&nbsp;') .. (value < 12 and ap[1] or ap[2])
    				end
    			end
    			if code.field == 'year' then
    				local sign = (era_text[eraopt] or {}).sign
    				if not sign or format:find('%{era}', 1, true) then
    					sign = ''
    					if value <= 0 then
    						value = 1 - value
    					end
    				else
    					if value >= 0 then
    						sign = ''
    					else
    						value = -value
    					end
    				end
    				return spaces .. sign .. sformat(fmt, value)
    			end
    			return spaces .. (fmt and sformat(fmt, value) or value)
    		end
    	end
    	local function replace_property(spaces, id)
    		if id == 'era' then
    			-- Special case so can use local era option.
    			local result = get_era_for_year(eraopt, date.year)
    			if result == '' then
    				return ''
    			end
    			return (spaces == '' and '' or '&nbsp;') .. result
    		end
    		local result = date[id]
    		if type(result) == 'string' then
    			return spaces .. result
    		end
    		if type(result) == 'number' then
    			return  spaces .. tostring(result)
    		end
    		if type(result) == 'boolean' then
    			return  spaces .. (result and '1' or '0')
    		end
    		-- This occurs if id is an undefined field in a partial date, or is the name of a function.
    		return nil
    	end
    	local PERCENT = '\127PERCENT\127'
    	return (format
    		:gsub('%%%%', PERCENT)
    		:gsub('(%s*)%%{(%w+)}', replace_property)
    		:gsub('(%s*)%%(%-?)(%a)', replace_code)
    		:gsub(PERCENT, '%%')
    	)
    end
    
    local function _date_text(date, fmt, options)
    	-- Return a formatted string representing the given date.
    	if not is_date(date) then
    		error('date:text: need a date (use "date:text()" with a colon)', 2)
    	end
    	if type(fmt) == 'string' and fmt:match('%S') then
    		if fmt:find('%', 1, true) then
    			return strftime(date, fmt, options)
    		end
    	elseif date.partial then
    		fmt = date.month and 'my' or 'y'
    	else
    		fmt = 'dmy'
    		if date.hastime then
    			fmt = (date.second > 0 and 'hms ' or 'hm ') .. fmt
    		end
    	end
    	local function bad_format()
    		-- For consistency with other format processing, return given format
    		-- (or cleaned format if original was not a string) if invalid.
    		return mw.text.nowiki(fmt)
    	end
    	if date.partial then
    		-- Ignore days in standard formats like 'ymd'.
    		if fmt == 'ym' or fmt == 'ymd' then
    			fmt = date.month and '%Y-%m %{era}' or '%Y %{era}'
    		elseif fmt == 'my' or fmt == 'dmy' or fmt == 'mdy' then
    			fmt = date.month and '%B %-Y %{era}' or '%-Y %{era}'
    		elseif fmt == 'y' then
    			fmt = date.month and '%-Y %{era}' or '%-Y %{era}'
    		else
    			return bad_format()
    		end
    		return strftime(date, fmt, options)
    	end
    	local function hm_fmt()
    		local plain = make_option_table(options, date.options).bydefault.am
    		return plain and '%H:%M' or '%-I:%M %p'
    	end
    	local need_time = date.hastime
    	local t = collection()
    	for item in fmt:gmatch('%S+') do
    		local f
    		if item == 'hm' then
    			f = hm_fmt()
    			need_time = false
    		elseif item == 'hms' then
    			f = '%H:%M:%S'
    			need_time = false
    		elseif item == 'ymd' then
    			f = '%Y-%m-%d %{era}'
    		elseif item == 'mdy' then
    			f = '%B %-d, %-Y %{era}'
    		elseif item == 'dmy' then
    			f = '%-d %B %-Y %{era}'
    		else
    			return bad_format()
    		end
    		t:add(f)
    	end
    	fmt = t:join(' ')
    	if need_time then
    		fmt = hm_fmt() .. ' ' .. fmt
    	end
    	return strftime(date, fmt, options)
    end
    
    local day_info = {
    	-- 0=Sun to 6=Sat
    	[0] = { 'Sun', 'Sunday' },
    	{ 'Mon', 'Monday' },
    	{ 'Tue', 'Tuesday' },
    	{ 'Wed', 'Wednesday' },
    	{ 'Thu', 'Thursday' },
    	{ 'Fri', 'Friday' },
    	{ 'Sat', 'Saturday' },
    }
    
    local month_info = {
    	-- 1=Jan to 12=Dec
    	{ 'Jan', 'January' },
    	{ 'Feb', 'February' },
    	{ 'Mar', 'March' },
    	{ 'Apr', 'April' },
    	{ 'May', 'May' },
    	{ 'Jun', 'June' },
    	{ 'Jul', 'July' },
    	{ 'Aug', 'August' },
    	{ 'Sep', 'September' },
    	{ 'Oct', 'October' },
    	{ 'Nov', 'November' },
    	{ 'Dec', 'December' },
    }
    
    local function name_to_number(text, translate)
    	if type(text) == 'string' then
    		return translate[text:lower()]
    	end
    end
    
    local function day_number(text)
    	return name_to_number(text, {
    		sun = 0, sunday = 0,
    		mon = 1, monday = 1,
    		tue = 2, tuesday = 2,
    		wed = 3, wednesday = 3,
    		thu = 4, thursday = 4,
    		fri = 5, friday = 5,
    		sat = 6, saturday = 6,
    	})
    end
    
    local function month_number(text)
    	return name_to_number(text, {
    		jan = 1, january = 1,
    		feb = 2, february = 2,
    		mar = 3, march = 3,
    		apr = 4, april = 4,
    		may = 5,
    		jun = 6, june = 6,
    		jul = 7, july = 7,
    		aug = 8, august = 8,
    		sep = 9, september = 9, sept = 9,
    		oct = 10, october = 10,
    		nov = 11, november = 11,
    		dec = 12, december = 12,
    	})
    end
    
    local function _list_text(list, fmt)
    	-- Return a list of formatted strings from a list of dates.
    	if not type(list) == 'table' then
    		error('date:list:text: need "list:text()" with a colon', 2)
    	end
    	local result = { join = _list_join }
    	for i, date in ipairs(list) do
    		result[i] = date:text(fmt)
    	end
    	return result
    end
    
    local function _date_list(date, spec)
    	-- Return a possibly empty numbered table of dates meeting the specification.
    	-- Dates in the list are in ascending order (oldest date first).
    	-- The spec should be a string of form "<count> <day> <op>"
    	-- where each item is optional and
    	--   count = number of items wanted in list
    	--   day = abbreviation or name such as Mon or Monday
    	--   op = >, >=, <, <= (default is > meaning after date)
    	-- If no count is given, the list is for the specified days in date's month.
    	-- The default day is date's day.
    	-- The spec can also be a positive or negative number:
    	--   -5 is equivalent to '5 <'
    	--   5  is equivalent to '5' which is '5 >'
    	if not is_date(date) then
    		error('date:list: need a date (use "date:list()" with a colon)', 2)
    	end
    	local list = { text = _list_text }
    	if date.partial then
    		return list
    	end
    	local count, offset, operation
    	local ops = {
    		['>='] = { before = false, include = true  },
    		['>']  = { before = false, include = false },
    		['<='] = { before = true , include = true  },
    		['<']  = { before = true , include = false },
    	}
    	if spec then
    		if type(spec) == 'number' then
    			count = floor(spec + 0.5)
    			if count < 0 then
    				count = -count
    				operation = ops['<']
    			end
    		elseif type(spec) == 'string' then
    			local num, day, op = spec:match('^%s*(%d*)%s*(%a*)%s*([<>=]*)%s*$')
    			if not num then
    				return list
    			end
    			if num ~= '' then
    				count = tonumber(num)
    			end
    			if day ~= '' then
    				local dow = day_number(day:gsub('[sS]$', ''))  -- accept plural days
    				if not dow then
    					return list
    				end
    				offset = dow - date.dow
    			end
    			operation = ops[op]
    		else
    			return list
    		end
    	end
    	offset = offset or 0
    	operation = operation or ops['>']
    	local datefrom, dayfirst, daylast
    	if operation.before then
    		if offset > 0 or (offset == 0 and not operation.include) then
    			offset = offset - 7
    		end
    		if count then
    			if count > 1 then
    				offset = offset - 7*(count - 1)
    			end
    			datefrom = date + offset
    		else
    			daylast = date.day + offset
    			dayfirst = daylast % 7
    			if dayfirst == 0 then
    				dayfirst = 7
    			end
    		end
    	else
    		if offset < 0 or (offset == 0 and not operation.include) then
    			offset = offset + 7
    		end
    		if count then
    			datefrom = date + offset
    		else
    			dayfirst = date.day + offset
    			daylast = date.monthdays
    		end
    	end
    	if not count then
    		if daylast < dayfirst then
    			return list
    		end
    		count = floor((daylast - dayfirst)/7) + 1
    		datefrom = Date(date, {day = dayfirst})
    	end
    	for i = 1, count do
    		if not datefrom then break end  -- exceeds date limits
    		list[i] = datefrom
    		datefrom = datefrom + 7
    	end
    	return list
    end
    
    -- A table to get the current date/time (UTC), but only if needed.
    local current = setmetatable({}, {
    	__index = function (self, key)
    		local d = os.date('!*t')
    		self.year = d.year
    		self.month = d.month
    		self.day = d.day
    		self.hour = d.hour
    		self.minute = d.min
    		self.second = d.sec
    		return rawget(self, key)
    	end })
    
    local function extract_date(newdate, text)
    	-- Parse the date/time in text and return n, o where
    	--   n = table of numbers with date/time fields
    	--   o = table of options for AM/PM or AD/BC or format, if any
    	-- or return nothing if date is known to be invalid.
    	-- Caller determines if the values in n are valid.
    	-- A year must be positive ('1' to '9999'); use 'BC' for BC.
    	-- In a y-m-d string, the year must be four digits to avoid ambiguity
    	-- ('0001' to '9999'). The only way to enter year <= 0 is by specifying
    	-- the date as three numeric parameters like ymd Date(-1, 1, 1).
    	-- Dates of form d/m/y, m/d/y, y/m/d are rejected as potentially ambiguous.
    	local date, options = {}, {}
    	if text:sub(-1) == 'Z' then
    		-- Extract date/time from a Wikidata timestamp.
    		-- The year can be 1 to 16 digits but this module handles 1 to 4 digits only.
    		-- Examples: '+2016-06-21T14:30:00Z', '-0000000180-00-00T00:00:00Z'.
    		local sign, y, m, d, H, M, S = text:match('^([+%-])(%d+)%-(%d%d)%-(%d%d)T(%d%d):(%d%d):(%d%d)Z$')
    		if sign then
    			y = tonumber(y)
    			if sign == '-' and y > 0 then
    				y = -y
    			end
    			if y <= 0 then
    				options.era = 'BCE'
    			end
    			date.year = y
    			m = tonumber(m)
    			d = tonumber(d)
    			H = tonumber(H)
    			M = tonumber(M)
    			S = tonumber(S)
    			if m == 0 then
    				newdate.partial = true
    				return date, options
    			end
    			date.month = m
    			if d == 0 then
    				newdate.partial = true
    				return date, options
    			end
    			date.day = d
    			if H > 0 or M > 0 or S > 0 then
    				date.hour = H
    				date.minute = M
    				date.second = S
    			end
    			return date, options
    		end
    		return
    	end
    	local function extract_ymd(item)
    		-- Called when no day or month has been set.
    		local y, m, d = item:match('^(%d%d%d%d)%-(%w+)%-(%d%d?)$')
    		if y then
    			if date.year then
    				return
    			end
    			if m:match('^%d%d?$') then
    				m = tonumber(m)
    			else
    				m = month_number(m)
    			end
    			if m then
    				date.year = tonumber(y)
    				date.month = m
    				date.day = tonumber(d)
    				return true
    			end
    		end
    	end
    	local function extract_day_or_year(item)
    		-- Called when a day would be valid, or
    		-- when a year would be valid if no year has been set and partial is set.
    		local number, suffix = item:match('^(%d%d?%d?%d?)(.*)$')
    		if number then
    			local n = tonumber(number)
    			if #number <= 2 and n <= 31 then
    				suffix = suffix:lower()
    				if suffix == '' or suffix == 'st' or suffix == 'nd' or suffix == 'rd' or suffix == 'th' then
    					date.day = n
    					return true
    				end
    			elseif suffix == '' and newdate.partial and not date.year then
    				date.year = n
    				return true
    			end
    		end
    	end
    	local function extract_month(item)
    		-- A month must be given as a name or abbreviation; a number could be ambiguous.
    		local m = month_number(item)
    		if m then
    			date.month = m
    			return true
    		end
    	end
    	local function extract_time(item)
    		local h, m, s = item:match('^(%d%d?):(%d%d)(:?%d*)$')
    		if date.hour or not h then
    			return
    		end
    		if s ~= '' then
    			s = s:match('^:(%d%d)$')
    			if not s then
    				return
    			end
    		end
    		date.hour = tonumber(h)
    		date.minute = tonumber(m)
    		date.second = tonumber(s)  -- nil if empty string
    		return true
    	end
    	local item_count = 0
    	local index_time
    	local function set_ampm(item)
    		local H = date.hour
    		if H and not options.am and index_time + 1 == item_count then
    			options.am = ampm_options[item]  -- caller checked this is not nil
    			if item:match('^[Aa]') then
    				if not (1 <= H and H <= 12) then
    					return
    				end
    				if H == 12 then
    					date.hour = 0
    				end
    			else
    				if not (1 <= H and H <= 23) then
    					return
    				end
    				if H <= 11 then
    					date.hour = H + 12
    				end
    			end
    			return true
    		end
    	end
    	for item in text:gsub(',', ' '):gsub('&nbsp;', ' '):gmatch('%S+') do
    		item_count = item_count + 1
    		if era_text[item] then
    			-- Era is accepted in peculiar places.
    			if options.era then
    				return
    			end
    			options.era = item
    		elseif ampm_options[item] then
    			if not set_ampm(item) then
    				return
    			end
    		elseif item:find(':', 1, true) then
    			if not extract_time(item) then
    				return
    			end
    			index_time = item_count
    		elseif date.day and date.month then
    			if date.year then
    				return  -- should be nothing more so item is invalid
    			end
    			if not item:match('^(%d%d?%d?%d?)$') then
    				return
    			end
    			date.year = tonumber(item)
    		elseif date.day then
    			if not extract_month(item) then
    				return
    			end
    		elseif date.month then
    			if not extract_day_or_year(item) then
    				return
    			end
    		elseif extract_month(item) then
    			options.format = 'mdy'
    		elseif extract_ymd(item) then
    			options.format = 'ymd'
    		elseif extract_day_or_year(item) then
    			if date.day then
    				options.format = 'dmy'
    			end
    		else
    			return
    		end
    	end
    	if not date.year or date.year == 0 then
    		return
    	end
    	local era = era_text[options.era]
    	if era and era.isbc then
    		date.year = 1 - date.year
    	end
    	return date, options
    end
    
    local function autofill(date1, date2)
    	-- Fill any missing month or day in each date using the
    	-- corresponding component from the other date, if present,
    	-- or with 1 if both dates are missing the month or day.
    	-- This gives a good result for calculating the difference
    	-- between two partial dates when no range is wanted.
    	-- Return filled date1, date2 (two full dates).
    	local function filled(a, b)
    		-- Return date a filled, if necessary, with month and/or day from date b.
    		-- The filled day is truncated to fit the number of days in the month.
    		local fillmonth, fillday
    		if not a.month then
    			fillmonth = b.month or 1
    		end
    		if not a.day then
    			fillday = b.day or 1
    		end
    		if fillmonth or fillday then  -- need to create a new date
    			a = Date(a, {
    				month = fillmonth,
    				day = math.min(fillday or a.day, days_in_month(a.year, fillmonth or a.month, a.calendar))
    			})
    		end
    		return a
    	end
    	return filled(date1, date2), filled(date2, date1)
    end
    
    local function date_add_sub(lhs, rhs, is_sub)
    	-- Return a new date from calculating (lhs + rhs) or (lhs - rhs),
    	-- or return nothing if invalid.
    	-- The result is nil if the calculated date exceeds allowable limits.
    	-- Caller ensures that lhs is a date; its properties are copied for the new date.
    	if lhs.partial then
    		-- Adding to a partial is not supported.
    		-- Can subtract a date or partial from a partial, but this is not called for that.
    		return
    	end
    	local function is_prefix(text, word, minlen)
    		local n = #text
    		return (minlen or 1) <= n and n <= #word and text == word:sub(1, n)
    	end
    	local function do_days(n)
    		local forcetime, jd
    		if floor(n) == n then
    			jd = lhs.jd
    		else
    			forcetime = not lhs.hastime
    			jd = lhs.jdz
    		end
    		jd = jd + (is_sub and -n or n)
    		if forcetime then
    			jd = tostring(jd)
    			if not jd:find('.', 1, true) then
    				jd = jd .. '.0'
    			end
    		end
    		return Date(lhs, 'juliandate', jd)
    	end
    	if type(rhs) == 'number' then
    		-- Add/subtract days, including fractional days.
    		return do_days(rhs)
    	end
    	if type(rhs) == 'string' then
    		-- rhs is a single component like '26m' or '26 months' (with optional sign).
    		-- Fractions like '3.25d' are accepted for the units which are handled as days.
    		local sign, numstr, id = rhs:match('^%s*([+-]?)([%d%.]+)%s*(%a+)$')
    		if sign then
    			if sign == '-' then
    				is_sub = not (is_sub and true or false)
    			end
    			local y, m, days
    			local num = tonumber(numstr)
    			if not num then
    				return
    			end
    			id = id:lower()
    			if is_prefix(id, 'years') then
    				y = num
    				m = 0
    			elseif is_prefix(id, 'months') then
    				y = floor(num / 12)
    				m = num % 12
    			elseif is_prefix(id, 'weeks') then
    				days = num * 7
    			elseif is_prefix(id, 'days') then
    				days = num
    			elseif is_prefix(id, 'hours') then
    				days = num / 24
    			elseif is_prefix(id, 'minutes', 3) then
    				days = num / (24 * 60)
    			elseif is_prefix(id, 'seconds') then
    				days = num / (24 * 3600)
    			else
    				return
    			end
    			if days then
    				return do_days(days)
    			end
    			if numstr:find('.', 1, true) then
    				return
    			end
    			if is_sub then
    				y = -y
    				m = -m
    			end
    			assert(-11 <= m and m <= 11)
    			y = lhs.year + y
    			m = lhs.month + m
    			if m > 12 then
    				y = y + 1
    				m = m - 12
    			elseif m < 1 then
    				y = y - 1
    				m = m + 12
    			end
    			local d = math.min(lhs.day, days_in_month(y, m, lhs.calendar))
    			return Date(lhs, y, m, d)
    		end
    	end
    	if is_diff(rhs) then
    		local days = rhs.age_days
    		if (is_sub or false) ~= (rhs.isnegative or false) then
    			days = -days
    		end
    		return lhs + days
    	end
    end
    
    local full_date_only = {
    	dayabbr = true,
    	dayname = true,
    	dow = true,
    	dayofweek = true,
    	dowiso = true,
    	dayofweekiso = true,
    	dayofyear = true,
    	gsd = true,
    	juliandate = true,
    	jd = true,
    	jdz = true,
    	jdnoon = true,
    }
    
    -- Metatable for a date's calculated fields.
    local datemt = {
    	__index = function (self, key)
    		if rawget(self, 'partial') then
    			if full_date_only[key] then return end
    			if key == 'monthabbr' or key == 'monthdays' or key == 'monthname' then
    				if not self.month then return end
    			end
    		end
    		local value
    		if key == 'dayabbr' then
    			value = day_info[self.dow][1]
    		elseif key == 'dayname' then
    			value = day_info[self.dow][2]
    		elseif key == 'dow' then
    			value = (self.jdnoon + 1) % 7  -- day-of-week 0=Sun to 6=Sat
    		elseif key == 'dayofweek' then
    			value = self.dow
    		elseif key == 'dowiso' then
    			value = (self.jdnoon % 7) + 1  -- ISO day-of-week 1=Mon to 7=Sun
    		elseif key == 'dayofweekiso' then
    			value = self.dowiso
    		elseif key == 'dayofyear' then
    			local first = Date(self.year, 1, 1, self.calendar).jdnoon
    			value = self.jdnoon - first + 1  -- day-of-year 1 to 366
    		elseif key == 'era' then
    			-- Era text (never a negative sign) from year and options.
    			value = get_era_for_year(self.options.era, self.year)
    		elseif key == 'format' then
    			value = self.options.format or 'dmy'
    		elseif key == 'gsd' then
    			-- GSD = 1 from 00:00:00 to 23:59:59 on 1 January 1 AD Gregorian calendar,
    			-- which is from jd 1721425.5 to 1721426.49999.
    			value = floor(self.jd - 1721424.5)
    		elseif key == 'juliandate' or key == 'jd' or key == 'jdz' then
    			local jd, jdz = julian_date(self)
    			rawset(self, 'juliandate', jd)
    			rawset(self, 'jd', jd)
    			rawset(self, 'jdz', jdz)
    			return key == 'jdz' and jdz or jd
    		elseif key == 'jdnoon' then
    			-- Julian date at noon (an integer) on the calendar day when jd occurs.
    			value = floor(self.jd + 0.5)
    		elseif key == 'isleapyear' then
    			value = is_leap_year(self.year, self.calendar)
    		elseif key == 'monthabbr' then
    			value = month_info[self.month][1]
    		elseif key == 'monthdays' then
    			value = days_in_month(self.year, self.month, self.calendar)
    		elseif key == 'monthname' then
    			value = month_info[self.month][2]
    		end
    		if value ~= nil then
    			rawset(self, key, value)
    			return value
    		end
    	end,
    }
    
    -- Date operators.
    local function mt_date_add(lhs, rhs)
    	if not is_date(lhs) then
    		lhs, rhs = rhs, lhs  -- put date on left (it must be a date for this to have been called)
    	end
    	return date_add_sub(lhs, rhs)
    end
    
    local function mt_date_sub(lhs, rhs)
    	if is_date(lhs) then
    		if is_date(rhs) then
    			return DateDiff(lhs, rhs)
    		end
    		return date_add_sub(lhs, rhs, true)
    	end
    end
    
    local function mt_date_concat(lhs, rhs)
    	return tostring(lhs) .. tostring(rhs)
    end
    
    local function mt_date_tostring(self)
    	return self:text()
    end
    
    local function mt_date_eq(lhs, rhs)
    	-- Return true if dates identify same date/time where, for example,
    	-- Date(-4712, 1, 1, 'Julian') == Date(-4713, 11, 24, 'Gregorian') is true.
    	-- This is called only if lhs and rhs have the same type and the same metamethod.
    	if lhs.partial or rhs.partial then
    		-- One date is partial; the other is a partial or a full date.
    		-- The months may both be nil, but must be the same.
    		return lhs.year == rhs.year and lhs.month == rhs.month and lhs.calendar == rhs.calendar
    	end
    	return lhs.jdz == rhs.jdz
    end
    
    local function mt_date_lt(lhs, rhs)
    	-- Return true if lhs < rhs, for example,
    	-- Date('1 Jan 2016') < Date('06:00 1 Jan 2016') is true.
    	-- This is called only if lhs and rhs have the same type and the same metamethod.
    	if lhs.partial or rhs.partial then
    		-- One date is partial; the other is a partial or a full date.
    		if lhs.calendar ~= rhs.calendar then
    			return lhs.calendar == 'Julian'
    		end
    		if lhs.partial then
    			lhs = lhs.partial.first
    		end
    		if rhs.partial then
    			rhs = rhs.partial.first
    		end
    	end
    	return lhs.jdz < rhs.jdz
    end
    
    --[[ Examples of syntax to construct a date:
    Date(y, m, d, 'julian')             default calendar is 'gregorian'
    Date(y, m, d, H, M, S, 'julian')
    Date('juliandate', jd, 'julian')    if jd contains "." text output includes H:M:S
    Date('currentdate')
    Date('currentdatetime')
    Date('1 April 1995', 'julian')      parse date from text
    Date('1 April 1995 AD', 'julian')   using an era sets a flag to do the same for output
    Date('04:30:59 1 April 1995', 'julian')
    Date(date)                          copy of an existing date
    Date(date, t)                       same, updated with y,m,d,H,M,S fields from table t
    Date(t)                       		date with y,m,d,H,M,S fields from table t
    ]]
    function Date(...)  -- for forward declaration above
    	-- Return a table holding a date assuming a uniform calendar always applies
    	-- (proleptic Gregorian calendar or proleptic Julian calendar), or
    	-- return nothing if date is invalid.
    	-- A partial date has a valid year, however its month may be nil, and
    	-- its day and time fields are nil.
    	-- Field partial is set to false (if a full date) or a table (if a partial date).
    	local calendars = { julian = 'Julian', gregorian = 'Gregorian' }
    	local newdate = {
    		_id = uniq,
    		calendar = 'Gregorian',  -- default is Gregorian calendar
    		hastime = false,  -- true if input sets a time
    		hour = 0,  -- always set hour/minute/second so don't have to handle nil
    		minute = 0,
    		second = 0,
    		options = {},
    		list = _date_list,
    		subtract = function (self, rhs, options)
    			return DateDiff(self, rhs, options)
    		end,
    		text = _date_text,
    	}
    	local argtype, datetext, is_copy, jd_number, tnums
    	local numindex = 0
    	local numfields = { 'year', 'month', 'day', 'hour', 'minute', 'second' }
    	local numbers = {}
    	for _, v in ipairs({...}) do
    		v = strip_to_nil(v)
    		local vlower = type(v) == 'string' and v:lower() or nil
    		if v == nil then
    			-- Ignore empty arguments after stripping so modules can directly pass template parameters.
    		elseif calendars[vlower] then
    			newdate.calendar = calendars[vlower]
    		elseif vlower == 'partial' then
    			newdate.partial = true
    		elseif vlower == 'fix' then
    			newdate.want_fix = true
    		elseif is_date(v) then
    			-- Copy existing date (items can be overridden by other arguments).
    			if is_copy or tnums then
    				return
    			end
    			is_copy = true
    			newdate.calendar = v.calendar
    			newdate.partial = v.partial
    			newdate.hastime = v.hastime
    			newdate.options = v.options
    			newdate.year = v.year
    			newdate.month = v.month
    			newdate.day = v.day
    			newdate.hour = v.hour
    			newdate.minute = v.minute
    			newdate.second = v.second
    		elseif type(v) == 'table' then
    			if tnums then
    				return
    			end
    			tnums = {}
    			local tfields = { year=1, month=1, day=1, hour=2, minute=2, second=2 }
    			for tk, tv in pairs(v) do
    				if tfields[tk] then
    					tnums[tk] = tonumber(tv)
    				end
    				if tfields[tk] == 2 then
    					newdate.hastime = true
    				end
    			end
    		else
    			local num = tonumber(v)
    			if not num and argtype == 'setdate' and numindex == 1 then
    				num = month_number(v)
    			end
    			if num then
    				if not argtype then
    					argtype = 'setdate'
    				end
    				if argtype == 'setdate' and numindex < 6 then
    					numindex = numindex + 1
    					numbers[numfields[numindex]] = num
    				elseif argtype == 'juliandate' and not jd_number then
    					jd_number = num
    					if type(v) == 'string' then
    						if v:find('.', 1, true) then
    							newdate.hastime = true
    						end
    					elseif num ~= floor(num) then
    						-- The given value was a number. The time will be used
    						-- if the fractional part is nonzero.
    						newdate.hastime = true
    					end
    				else
    					return
    				end
    			elseif argtype then
    				return
    			elseif type(v) == 'string' then
    				if v == 'currentdate' or v == 'currentdatetime' or v == 'juliandate' then
    					argtype = v
    				else
    					argtype = 'datetext'
    					datetext = v
    				end
    			else
    				return
    			end
    		end
    	end
    	if argtype == 'datetext' then
    		if tnums or not set_date_from_numbers(newdate, extract_date(newdate, datetext)) then
    			return
    		end
    	elseif argtype == 'juliandate' then
    		newdate.partial = nil
    		newdate.jd = jd_number
    		if not set_date_from_jd(newdate) then
    			return
    		end
    	elseif argtype == 'currentdate' or argtype == 'currentdatetime' then
    		newdate.partial = nil
    		newdate.year = current.year
    		newdate.month = current.month
    		newdate.day = current.day
    		if argtype == 'currentdatetime' then
    			newdate.hour = current.hour
    			newdate.minute = current.minute
    			newdate.second = current.second
    			newdate.hastime = true
    		end
    		newdate.calendar = 'Gregorian'  -- ignore any given calendar name
    	elseif argtype == 'setdate' then
    		if tnums or not set_date_from_numbers(newdate, numbers) then
    			return
    		end
    	elseif not (is_copy or tnums) then
    		return
    	end
    	if tnums then
    		newdate.jd = nil  -- force recalculation in case jd was set before changes from tnums
    		if not set_date_from_numbers(newdate, tnums) then
    			return
    		end
    	end
    	if newdate.partial then
    		local year = newdate.year
    		local month = newdate.month
    		local first = Date(year, month or 1, 1, newdate.calendar)
    		month = month or 12
    		local last = Date(year, month, days_in_month(year, month), newdate.calendar)
    		newdate.partial = { first = first, last = last }
    	else
    		newdate.partial = false  -- avoid index lookup
    	end
    	setmetatable(newdate, datemt)
    	local readonly = {}
    	local mt = {
    		__index = newdate,
    		__newindex = function(t, k, v) error('date.' .. tostring(k) .. ' is read-only', 2) end,
    		__add = mt_date_add,
    		__sub = mt_date_sub,
    		__concat = mt_date_concat,
    		__tostring = mt_date_tostring,
    		__eq = mt_date_eq,
    		__lt = mt_date_lt,
    	}
    	return setmetatable(readonly, mt)
    end
    
    local function _diff_age(diff, code, options)
    	-- Return a tuple of integer values from diff as specified by code, except that
    	-- each integer may be a list of two integers for a diff with a partial date, or
    	-- return nil if the code is not supported.
    	-- If want round, the least significant unit is rounded to nearest whole unit.
    	-- For a duration, an extra day is added.
    	local wantround, wantduration, wantrange
    	if type(options) == 'table' then
    		wantround = options.round
    		wantduration = options.duration
    		wantrange = options.range
    	else
    		wantround = options
    	end
    	if not is_diff(diff) then
    		local f = wantduration and 'duration' or 'age'
    		error(f .. ': need a date difference (use "diff:' .. f .. '()" with a colon)', 2)
    	end
    	if diff.partial then
    		-- Ignore wantround, wantduration.
    		local function choose(v)
    			if type(v) == 'table' then
    				if not wantrange or v[1] == v[2] then
    					-- Example: Date('partial', 2005) - Date('partial', 2001) gives
    					-- diff.years = { 3, 4 } to show the range of possible results.
    					-- If do not want a range, choose the second value as more expected.
    					return v[2]
    				end
    			end
    			return v
    		end
    		if code == 'ym' or code == 'ymd' then
    			if not wantrange and diff.iszero then
    				-- This avoids an unexpected result such as
    				-- Date('partial', 2001) - Date('partial', 2001)
    				-- giving diff = { years = 0, months = { 0, 11 } }
    				-- which would be reported as 0 years and 11 months.
    				return 0, 0
    			end
    			return choose(diff.partial.years), choose(diff.partial.months)
    		end
    		if code == 'y' then
    			return choose(diff.partial.years)
    		end
    		if code == 'm' or code == 'w' or code == 'd' then
    			return choose({ diff.partial.mindiff:age(code), diff.partial.maxdiff:age(code) })
    		end
    		return nil
    	end
    	local extra_days = wantduration and 1 or 0
    	if code == 'wd' or code == 'w' or code == 'd' then
    		local offset = wantround and 0.5 or 0
    		local days = diff.age_days + extra_days
    		if code == 'wd' or code == 'd' then
    			days = floor(days + offset)
    			if code == 'd' then
    				return days
    			end
    			return floor(days/7), days % 7
    		end
    		return floor(days/7 + offset)
    	end
    	local H, M, S = diff.hours, diff.minutes, diff.seconds
    	if code == 'dh' or code == 'dhm' or code == 'dhms' or code == 'h' or code == 'hm' or code == 'hms' or code == 'M' or code == 's' then
    		local days = floor(diff.age_days + extra_days)
    		local inc_hour
    		if wantround then
    			if code == 'dh' or code == 'h' then
    				if M >= 30 then
    					inc_hour = true
    				end
    			elseif code == 'dhm' or code == 'hm' then
    				if S >= 30 then
    					M = M + 1
    					if M >= 60 then
    						M = 0
    						inc_hour = true
    					end
    				end
    			elseif code == 'M' then
    				if S >= 30 then
    					M = M + 1
    				end
    			else
    				-- Nothing needed because S is an integer.
    			end
    			if inc_hour then
    				H = H + 1
    				if H >= 24 then
    					H = 0
    					days = days + 1
    				end
    			end
    		end
    		if code == 'dh' or code == 'dhm' or code == 'dhms' then
    			if code == 'dh' then
    				return days, H
    			elseif code == 'dhm' then
    				return days, H, M
    			else
    				return days, H, M, S
    			end
    		end
    		local hours = days * 24 + H
    		if code == 'h' then
    			return hours
    		elseif code == 'hm' then
    			return hours, M
    		elseif code == 'M' or code == 's' then
    			M = hours * 60 + M
    			if code == 'M' then
    				return M
    			end
    			return M * 60 + S
    		end
    		return hours, M, S
    	end
    	if wantround then
    		local inc_hour
    		if code == 'ymdh' or code == 'ymwdh' then
    			if M >= 30 then
    				inc_hour = true
    			end
    		elseif code == 'ymdhm' or code == 'ymwdhm' then
    			if S >= 30 then
    				M = M + 1
    				if M >= 60 then
    					M = 0
    					inc_hour = true
    				end
    			end
    		elseif code == 'ymd' or code == 'ymwd' or code == 'yd' or code == 'md' then
    			if H >= 12 then
    				extra_days = extra_days + 1
    			end
    		end
    		if inc_hour then
    			H = H + 1
    			if H >= 24 then
    				H = 0
    				extra_days = extra_days + 1
    			end
    		end
    	end
    	local y, m, d = diff.years, diff.months, diff.days
    	if extra_days > 0 then
    		d = d + extra_days
    		if d > 28 or code == 'yd' then
    			-- Recalculate in case have passed a month.
    			diff = diff.date1 + extra_days - diff.date2
    			y, m, d = diff.years, diff.months, diff.days
    		end
    	end
    	if code == 'ymd' then
    		return y, m, d
    	elseif code == 'yd' then
    		if y > 0 then
    			-- It is known that diff.date1 > diff.date2.
    			diff = diff.date1 - (diff.date2 + (y .. 'y'))
    		end
    		return y, floor(diff.age_days)
    	elseif code == 'md' then
    		return y * 12 + m, d
    	elseif code == 'ym' or code == 'm' then
    		if wantround then
    			if d >= 16 then
    				m = m + 1
    				if m >= 12 then
    					m = 0
    					y = y + 1
    				end
    			end
    		end
    		if code == 'ym' then
    			return y, m
    		end
    		return y * 12 + m
    	elseif code == 'ymw' then
    		local weeks = floor(d/7)
    		if wantround then
    			local days = d % 7
    			if days > 3 or (days == 3 and H >= 12) then
    				weeks = weeks + 1
    			end
    		end
    		return y, m, weeks
    	elseif code == 'ymwd' then
    		return y, m, floor(d/7), d % 7
    	elseif code == 'ymdh' then
    		return y, m, d, H
    	elseif code == 'ymwdh' then
    		return y, m, floor(d/7), d % 7, H
    	elseif code == 'ymdhm' then
    		return y, m, d, H, M
    	elseif code == 'ymwdhm' then
    		return y, m, floor(d/7), d % 7, H, M
    	end
    	if code == 'y' then
    		if wantround and m >= 6 then
    			y = y + 1
    		end
    		return y
    	end
    	return nil
    end
    
    local function _diff_duration(diff, code, options)
    	if type(options) ~= 'table' then
    		options = { round = options }
    	end
    	options.duration = true
    	return _diff_age(diff, code, options)
    end
    
    -- Metatable for some operations on date differences.
    diffmt = {  -- for forward declaration above
    	__concat = function (lhs, rhs)
    		return tostring(lhs) .. tostring(rhs)
    	end,
    	__tostring = function (self)
    		return tostring(self.age_days)
    	end,
    	__index = function (self, key)
    		local value
    		if key == 'age_days' then
    			if rawget(self, 'partial') then
    				local function jdz(date)
    					return (date.partial and date.partial.first or date).jdz
    				end
    				value = jdz(self.date1) - jdz(self.date2)
    			else
    				value = self.date1.jdz - self.date2.jdz
    			end
    		end
    		if value ~= nil then
    			rawset(self, key, value)
    			return value
    		end
    	end,
    }
    
    function DateDiff(date1, date2, options)  -- for forward declaration above
    	-- Return a table with the difference between two dates (date1 - date2).
    	-- The difference is negative if date1 is older than date2.
    	-- Return nothing if invalid.
    	-- If d = date1 - date2 then
    	--     date1 = date2 + d
    	-- If date1 >= date2 and the dates have no H:M:S time specified then
    	--     date1 = date2 + (d.years..'y') + (d.months..'m') + d.days
    	-- where the larger time units are added first.
    	-- The result of Date(2015,1,x) + '1m' is Date(2015,2,28) for
    	-- x = 28, 29, 30, 31. That means, for example,
    	--     d = Date(2015,3,3) - Date(2015,1,31)
    	-- gives d.years, d.months, d.days = 0, 1, 3 (excluding date1).
    	if not (is_date(date1) and is_date(date2) and date1.calendar == date2.calendar) then
    		return
    	end
    	local wantfill
    	if type(options) == 'table' then
    		wantfill = options.fill
    	end
    	local isnegative = false
    	local iszero = false
    	if date1 < date2 then
    		isnegative = true
    		date1, date2 = date2, date1
    	elseif date1 == date2 then
    		iszero = true
    	end
    	-- It is known that date1 >= date2 (period is from date2 to date1).
    	if date1.partial or date2.partial then
    		-- Two partial dates might have timelines:
    		---------------------A=================B--- date1 is from A to B inclusive
    		--------C=======D-------------------------- date2 is from C to D inclusive
    		-- date1 > date2 iff A > C (date1.partial.first > date2.partial.first)
    		-- The periods can overlap ('April 2001' - '2001'):
    		-------------A===B------------------------- A=2001-04-01  B=2001-04-30
    		--------C=====================D------------ C=2001-01-01  D=2001-12-31
    		if wantfill then
    			date1, date2 = autofill(date1, date2)
    		else
    			local function zdiff(date1, date2)
    				local diff = date1 - date2
    				if diff.isnegative then
    					return date1 - date1  -- a valid diff in case we call its methods
    				end
    				return diff
    			end
    			local function getdate(date, which)
    				return date.partial and date.partial[which] or date
    			end
    			local maxdiff = zdiff(getdate(date1, 'last'), getdate(date2, 'first'))
    			local mindiff = zdiff(getdate(date1, 'first'), getdate(date2, 'last'))
    			local years, months
    			if maxdiff.years == mindiff.years then
    				years = maxdiff.years
    				if maxdiff.months == mindiff.months then
    					months = maxdiff.months
    				else
    					months = { mindiff.months, maxdiff.months }
    				end
    			else
    				years = { mindiff.years, maxdiff.years }
    			end
    			return setmetatable({
    				date1 = date1,
    				date2 = date2,
    				partial = {
    					years = years,
    					months = months,
    					maxdiff = maxdiff,
    					mindiff = mindiff,
    				},
    				isnegative = isnegative,
    				iszero = iszero,
    				age = _diff_age,
    				duration = _diff_duration,
    			}, diffmt)
    		end
    	end
    	local y1, m1 = date1.year, date1.month
    	local y2, m2 = date2.year, date2.month
    	local years = y1 - y2
    	local months = m1 - m2
    	local d1 = date1.day + hms(date1)
    	local d2 = date2.day + hms(date2)
    	local days, time
    	if d1 >= d2 then
    		days = d1 - d2
    	else
    		months = months - 1
    		-- Get days in previous month (before the "to" date) given December has 31 days.
    		local dpm = m1 > 1 and days_in_month(y1, m1 - 1, date1.calendar) or 31
    		if d2 >= dpm then
    			days = d1 - hms(date2)
    		else
    			days = dpm - d2 + d1
    		end
    	end
    	if months < 0 then
    		years = years - 1
    		months = months + 12
    	end
    	days, time = math.modf(days)
    	local H, M, S = h_m_s(time)
    	return setmetatable({
    		date1 = date1,
    		date2 = date2,
    		partial = false,  -- avoid index lookup
    		years = years,
    		months = months,
    		days = days,
    		hours = H,
    		minutes = M,
    		seconds = S,
    		isnegative = isnegative,
    		iszero = iszero,
    		age = _diff_age,
    		duration = _diff_duration,
    	}, diffmt)
    end
    
    return {
    	_current = current,
    	_Date = Date,
    	_days_in_month = days_in_month,
    }