haskell - 如何判断现在是夏季还是冬季？

Question

我想知道是否在某个Day上(package time) 现在是夏令时或冬令时。

我对中欧特别感兴趣，那里夏季时区为 CEST = GMT+2，冬季时区为 CEST = GMT+1。

我需要它，因为我需要转换 JulianDate (包astro)到本地时间并返回。

Answer 1

由于时区更改的确切日期因地点而异，我认为您必须编写自己的函数来检查这一点。

我碰巧知道欧洲夏令时从三月的最后一个星期日开始，到十月的最后一个星期日结束。我在下面的函数中使用了它，很容易更改为其他位置。

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