--[[
Calculates the Gregorian date of a recurring holiday that varies year-to-year, but follows the rule "Nth [day of week] of [month]"
"month" = month number (1 to 12)
"weeknumber" = number of week (1 to 4, or -1 to mean "last")
"dayofweek" = number that represents the day of the week, where 1 = Sunday and 7 = Saturday
"year" = Gregorian calendar year
]]
require('strict')
local p = {}
function p.calculate(frame)
local args = frame.args
local ONE_DAY = 86400 -- number of seconds in one day
local year = tonumber(args.year)
local month = tonumber(args.month)
local weeknumber = tonumber(args.weeknumber)
local dayofweek = tonumber(args.dayofweek)
local date = os.time{year=year, month=month, day=1}
local dateparts = os.date("*t", date)
if weeknumber > 0 then
-- find the first [dayofweek] of this month
while dateparts["wday"] ~= dayofweek do
date = date + ONE_DAY
dateparts = os.date("*t", date)
end
-- add the correct number of weeks
if weeknumber > 1 then
date = date + ((weeknumber - 1) * (7 * ONE_DAY))
end
else
-- find the first day of the next month
while dateparts["month"] == month do
date = date + ONE_DAY
dateparts = os.date("*t", date)
end
-- go back one day to get the last day of the month we want
date = date - ONE_DAY
dateparts = os.date("*t", date)
-- go backwards until we find the right day of week
while dateparts["wday"] ~= dayofweek do
date = date - ONE_DAY
dateparts = os.date("*t", date)
end
end
local result = os.date("%Y-%m-%d", date)
return result
end
return p
