Elm:
计算未来或过去的日期

import Time
import Date exposing (Date)

-- 创建日期: 2023年3月1日
fromDate : Date
fromDate = Date.fromParts 2023 Date.March 1

-- 计算未来日期: 10天后
calculateFutureDate : Date -> Int -> Date
calculateFutureDate date daysToAdd =
    Time.posixToMillis (Date.toPosix date) + daysToAdd * 86400000
    |> Time.millisToPosix
    |> Date.fromPosix

futureDate : Date
futureDate = calculateFutureDate fromDate 10
-- 输出: Date { year = 2023, month = Date.March, day = 11 }

-- 计算过去日期: 10天前
calculatePastDate : Date -> Int -> Date
calculatePastDate date daysToSubtract =
    Time.posixToMillis (Date.toPosix date) - daysToSubtract * 86400000
    |> Time.millisToPosix
    |> Date.fromPosix

pastDate : Date
pastDate = calculatePastDate fromDate 10
-- 输出: Date { year = 2023, month = Date.February, day = 19 }