Module: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 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.
	-- Otherwise return text (convenient when Date fields are provided from
	-- another module which is able to pass, for example, a number).
	if type(text) == 'string' then
		text = text:match('(%S.-)%s*$')
	end
	return text
end

local function number_name(number, singular, plural, sep)
	-- Return the given number, converted to a string, with the
	-- separator (default space) and singular or plural name appended.
	plural = plural or (singular .. 's')
	sep = sep or ' '
	return tostring(number) .. sep .. ((number == 1) and singular or plural)
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 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 floor = math.floor
	local offset
	local a = floor((14 - date.month)/12)
	local y = date.year + 4800 - a
	if date.calname == '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 + (date.hour + (date.minute + date.second / 60) /60) / 24 - 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 floor = math.floor
	local calname = date.calname
	local limits  -- min/max limits for date ranges −9999-01-01 to 9999-12-31
	if calname == 'Julian' then
		limits = { -1931076.5, 5373557.49999 }
	elseif calname == 'Gregorian' then
		limits = { -1930999.5, 5373484.49999 }
	else
		return
	end
	local jd = date.jd
	if not (type(jd) == 'number' and limits[1] <= jd and jd <= limits[2]) then
		return
	end
	local jdn = floor(jd)
	if date.hastime then
		local time = jd - jdn
		local hour
		if time >= 0.5 then
			jdn = jdn + 1
			time = time - 0.5
			hour = 0
		else
			hour = 12
		end
		time = floor(time * 24 * 3600 + 0.5)  -- number of seconds after hour
		date.second = time % 60
		time = floor(time / 60)
		date.minute = time % 60
		date.hour = hour + floor(time / 60)
	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 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
	if not (y and m and d) or not
		(-9999 <= y and y <= 9999 and
			1 <= m and m <= 12 and
			1 <= d and d <= days_in_month(y, m, date.calname)) then
		return
	end
	if H then
		date.hastime = true
	else
		H = date.hour or 0
	end
	if not (0 <= H and H <= 23 and
			0 <= M and M <= 59 and
			0 <= S and S <= 59) then
		return
	end
	date.year = y    -- -9999 to 9999 ('n BC' → year = 1 - n)
	date.month = m   -- 1 to 12
	date.day = d     -- 1 to 31
	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' }) 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 options2 or defaults.
	-- Valid option settings are:
	-- am: 'am', 'a.m.', 'AM', 'A.M.'
	-- 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.
	-- 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 = {}
	if type(options1) == 'table' then
		result = options1
	elseif type(options1) == 'string' then
		-- Example: '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
		result[k] = result[k] or options2[k] or v
	end
	return result
end

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 'BC'] or {})[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 = 'doy' },
		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]
			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.' },
					})[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, for example, if id 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
		return 'Need a date (use "date:text()" with a colon).'
	end
	if type(fmt) ~= 'string' then
		fmt = '%-d %B %-Y %{era}'
		if date.hastime then
			if date.second > 0 then
				fmt = '%H:%M:%S ' .. fmt
			else
				fmt = '%H:%M ' .. fmt
			end
		end
		return strftime(date, fmt, options)
	end
	if fmt:find('%', 1, true) then
		return strftime(date, fmt, options)
	end
	local t = collection()
	for item in fmt:gmatch('%S+') do
		local f
		if item == 'hm' then
			f = '%H:%M'
		elseif item == 'hms' then
			f = '%H:%M:%S'
		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 '(invalid format)'
		end
		t:add(f)
	end
	return strftime(date, t:join(' '), 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,
		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
		return 'Need "list:text()" with a colon.'
	end
	local result = { join = _list_join }
	for i, date in ipairs(list) do
		result[i] = date:text(fmt)
	end
	return result
end

local function _make_list(date, spec)
	-- Return a possibly empty numbered table of dates meeting the specification.
	-- The spec should be a string like "Tue >=" meaning that the list will
	-- hold dates for all Tuesdays on or after date, and in date's month.
	if not is_date(date) then
		return 'Need a date (use "date:list()" with a colon).'
	end
	local want_dow, op
	local ops = {
		['>='] = { before = false, include = true  },
		['>']  = { before = false, include = false },
		['<='] = { before = true , include = true  },
		['<']  = { before = true , include = false },
	}
	if spec then
		if type(spec) ~= 'string' then
			return {}
		end
		for item in spec:gmatch('%S+') do
			if ops[item] then
				if op then
					return {}
				end
				op = ops[item]
			else
				local dow = day_number(item)
				if dow then
					if want_dow then
						-- LATER Could handle more than one day, but probably not needed.
						return {}
					end
					want_dow = dow
				else
					return {}
				end
			end
		end
	end
	local offset = want_dow and want_dow - date.dow or 0
	op = op or ops['>=']
	local first, last
	if op.before then
		if offset >= 0 and not (op.include and offset == 0) then
			offset = offset - 7
		end
		last = date.day + offset
		first = last % 7
		if first == 0 then
			first = 7
		end
	else
		if offset < 0 or (not op.include and offset == 0) then
			offset = offset + 7
		end
		first = date.day + offset
		last = date.monthdays
	end
	local list = { text = _list_text }
	local count = math.floor((last - first)/7) + 1
	for i = 1, count do
		list[i] = Date(date, {day = first})
		first = first + 7
	end
	return list
end

-- A table to get the current date/time (UTC), but only if needed.
-- A local test can set the global variable to produce fixed results.
local current = setmetatable(
	set_current_for_test or {}, {
	__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(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, 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 ambiguous.
	local date, options = {}, {}
	local function extract_ymd(item)
		local ystr, mstr, dstr = item:match('^(%d%d%d%d)%-(%w+)%-(%d%d?)$')
		if ystr then
			local m
			if mstr:match('^%d%d?$') then
				m = tonumber(mstr)
			else
				m = month_number(mstr)
			end
			if m then
				date.year = tonumber(ystr)
				date.month = m
				date.day = tonumber(dstr)
				return true
			end
		end
	end
	local function extract_month(item)
		-- A month must be given as a name or abbreviation; a number would 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 ampm_options = {
		['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 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]
			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(',', ' '):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?)$') then
				return
			end
			date.year = tonumber(item)
		elseif date.day then
			if not extract_month(item) then
				return
			end
		elseif date.month then
			if not item:match('^(%d%d?)$') then
				return
			end
			date.day = tonumber(item)
		elseif not extract_ymd(item) then
			if item:match('^(%d%d?)$') then
				date.day = tonumber(item)
			elseif not extract_month(item) then
				return
			end
		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 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.
	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 math.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' (unsigned integer only).
		local num, id = rhs:match('^%s*(%d+)%s*(%a+)$')
		if num then
			local y, m
			num = tonumber(num)
			id = id:lower()
			if is_prefix(id, 'years') then
				y = num
				m = 0
			elseif is_prefix(id, 'months') then
				y = math.floor(num / 12)
				m = num % 12
			elseif is_prefix(id, 'weeks') then
				return do_days(num * 7)
			elseif is_prefix(id, 'days') then
				return do_days(num)
			elseif is_prefix(id, 'hours') then
				return do_days(num / 24)
			elseif is_prefix(id, 'minutes', 3) then
				return do_days(num / (24 * 60))
			elseif is_prefix(id, 'seconds') then
				return do_days(num / (24 * 3600))
			else
				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.calname))
			return Date(lhs, y, m, d)
		end
	end
end

-- Metatable for a date's calculated fields.
local datemt = {
	__index = function (self, key)
		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 == 'doy' then
			local first = Date(self.year, 1, 1, self.calname).jdnoon
			value = self.jdnoon - first + 1  -- day-of-year 1 to 366
		elseif key == 'dayofyear' then
			value = self.doy
		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 == '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 = math.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 = math.floor(self.jd + 0.5)
		elseif key == 'isleapyear' then
			value = is_leap_year(self.year, self.calname)
		elseif key == 'monthabbr' then
			value = month_info[self.month][1]
		elseif key == 'monthdays' then
			value = days_in_month(self.year, self.month, self.calname)
		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 only called if lhs and rhs have the same metatable.
	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 only called if lhs and rhs have the same metatable.
	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.
	local calendars = { julian = 'Julian', gregorian = 'Gregorian' }
	local newdate = {
		_id = uniq,
		calname = '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 = make_option_table(),
		list = _make_list,
		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.calname = calendars[vlower]
		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.calname = v.calname
			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 ~= math.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(datetext)) then
			return
		end
	elseif argtype == 'juliandate' then
		newdate.jd = jd_number
		if not set_date_from_jd(newdate) then
			return
		end
	elseif argtype == 'currentdate' or argtype == 'currentdatetime' then
		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.calname = '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
	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 _age_ym(diff)
	-- Return text specifying date difference in years, months.
	local sign = diff.isnegative and MINUS or ''
	local mtext = number_name(diff.months, 'month')
	local result
	if diff.years > 0 then
		local ytext = number_name(diff.years, 'year')
		if diff.months == 0 then
			result = ytext
		else
			result = ytext .. ',&nbsp;' .. mtext
		end
	else
		if diff.months == 0 then
			sign = ''
		end
		result = mtext
	end
	return sign .. result
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.daystotal)
	end,
	__index = function (self, key)
		local value
		if key == 'age_ym' then
			value = _age_ym(self)
		elseif key == 'daystotal' then
			value = self.date1.jdz - self.date2.jdz
		end
		if value ~= nil then
			rawset(self, key, value)
			return value
		end
	end,
}

function DateDiff(date1, date2)  -- for forward declaration above
	-- Return a table with the difference between the two dates (date1 - date2).
	-- The difference is negative if date1 is older than date2.
	-- Return nothing if invalid.
	if not (is_date(date1) and is_date(date2) and date1.calname == date2.calname) then
		return
	end
	local isnegative = false
	if date1 < date2 then
		isnegative = true
		date1, date2 = date2, date1
	end
	-- It is known that date1 >= date2.
	local y1, m1 = date1.year, date1.month
	local y2, m2 = date2.year, date2.month
	local years, months, days = y1 - y2, m1 - m2, date1.day - date2.day
	if days < 0 then
		days = days + days_in_month(y2, m2, date2.calname)
		months = months - 1
	end
	if months < 0 then
		months = months + 12
		years = years - 1
	end
	return setmetatable({
		date1 = date1,
		date2 = date2,
		years = years,
		months = months,
		days = days,
		isnegative = isnegative,
	}, diffmt)
end

return {
	_current = current,
	_Date = Date,
	_days_in_month = days_in_month,
}