# Data/Time/Calendar/JulianYearDay.hs

    -- #hide
    module Data.Time.Calendar.JulianYearDay
    	(
    	-- * Year and day format
    	module Data.Time.Calendar.JulianYearDay
    	) where

    import Data.Time.Calendar.Days
    import Data.Time.Calendar.Private

    -- | convert to proleptic Julian year and day format. First element of result is year (proleptic Julian calendar),
    -- second is the day of the year, with 1 for Jan 1, and 365 (or 366 in leap years) for Dec 31.
    toJulianYearAndDay :: Day -> (Integer,Int)
    toJulianYearAndDay (ModifiedJulianDay mjd) = (year,yd) where
    	a = mjd + 678577
    	quad = div a 1461
    	d = mod a 1461
    	y = min (div d 365) 3
    	yd = fromInteger (d - (y * 365) + 1)
    	year = quad * 4 + y + 1

    -- | convert from proleptic Julian year and day format.
    -- Invalid day numbers will be clipped to the correct range (1 to 365 or 366).
    fromJulianYearAndDay :: Integer -> Int -> Day
    fromJulianYearAndDay year day = ModifiedJulianDay mjd where
    	y = year - 1
    	mjd = (fromIntegral (clip 1 (if isJulianLeapYear year then 366 else 365) day)) + (365 * y) + (div y 4) - 678578

    -- | convert from proleptic Julian year and day format.
    -- Invalid day numbers will return Nothing
    fromJulianYearAndDayValid :: Integer -> Int -> Maybe Day
    fromJulianYearAndDayValid year day = do
    	day' <- clipValid 1 (if isJulianLeapYear year then 366 else 365) day
    	let
    		y = year - 1
    		mjd = (fromIntegral day') + (365 * y) + (div y 4) - 678578
    	return (ModifiedJulianDay mjd)

    -- | show in proleptic Julian year and day format (yyyy-ddd)
    showJulianYearAndDay :: Day -> String
    showJulianYearAndDay date = (show4 (Just '0') y) ++ "-" ++ (show3 (Just '0') d) where
    	(y,d) = toJulianYearAndDay date

    -- | Is this year a leap year according to the proleptic Julian calendar?
    isJulianLeapYear :: Integer -> Bool
    isJulianLeapYear year = (mod year 4 == 0)