haskell - 如何确定是夏季还是冬季？

我想确定某个特定的Day（包time）是夏季时间还是冬季时间。

我对中欧特别感兴趣，在欧洲中部，夏令时为CEST = GMT + 2，冬季为CEST = GMT + 1。

我需要它，因为我需要将JulianDate（包astro）转换为本地时间并返回。

最佳答案

因为时区更改的确切日期在每个地方都不同，所以我认为您必须编写自己的函数进行检查。

我碰巧知道在欧洲，夏季时间从3月的最后一个星期日开始，到10月的最后一个星期日结束。我在下面的函数中使用了它，很容易更改其他位置。

import Data.Time.Calendar
import Data.Time.Calendar.WeekDate
import Data.Tuple.Select

-- | Uses only days, it does not take the exact hour into account at which summer time starts and ends.
isSummerTime :: Day -- ^ Date to check if summer time is active
            -> Bool -- ^ Whether summer time is active
isSummerTime date = date > lastSundayMarch && date < lastSundayOctober
    where
        year = sel1 $ toGregorian date
        -- Find last Sunday in March
        aprilOne = fromGregorian year 4 1
        -- 1 is Monday, ..., 7 is Sunday
        aprilOneWeekDay = sel3 $ toWeekDate aprilOne
        -- Use the day number to find Sunday of the previous week: the last Sunday in March
        lastSundayMarch = addDays (-(toInteger aprilOneWeekDay)) aprilOne
        -- Same for end of summer time in October
        novemberOne = fromGregorian year 11 1
        novemberOneWeekDay = sel3 $ toWeekDate novemberOne
        lastSundayOctober = addDays (-(toInteger novemberOneWeekDay)) novemberOne

isSummerTime $ fromGregorian 2018 3 25 -- False
isSummerTime $ fromGregorian 2018 3 26 -- True
isSummerTime $ fromGregorian 2018 10 27 -- True
isSummerTime $ fromGregorian 2018 10 28 -- False