local dates = {}
local one_day = os.difftime(
	os.time{year=1970, month=1, day=2},
	os.time{year=1970, month=1, day=1})


function dates.days_in (year)
	if year % 4 ~= 0 or year % 100 == 0 and year % 400 ~= 0 then
		return 365
	end
	return 366
end


function dates.this_year ()
	return tonumber(os.date"%Y")
end


function dates.first_week_day (year)
	year = year or activity.this_year()
	return os.date("*t", os.time{year=year, month=1, day=1}).wday
end


function dates.day_of_year (date)
	local day = tonumber(os.date("%j", os.time(date)))
	if not day then
		error"invalid date"
	end
	return day
end


function dates.before (date, days)
	local time = os.time(date) - days * one_day
	date = os.date("*t", time)
	return {year=date.year, month=date.month, day=date.day}
end


return dates