Timezone Shenanigans

timezone-shenanigans.swift

import Foundation

let date1 = Date(timeIntervalSince1970: 0) // 1970-01-01 00:00:00
let date2 = Date(timeIntervalSince1970: 60 * 60 * 24)// 1970-01-02 00:00:00

func compareDates(date1: Date, date2: Date){
    switch date1 {
    case date2:
        print("date1 and date2 represent the same point in time")
    case ...date2:
        print("date1 is earlier in time than date2")
    case date2...:
        print("date1 is later in time than date2")
    default:
        return
    }
}
compareDates(date1: date1, date2: date2)

func startOfDayIn(date: Date, timeZone: TimeZone) -> Date {
    var calendar = Calendar.current
    calendar.timeZone = timeZone
    return calendar.startOfDay(for: date)
}

let date = Date(timeIntervalSince1970: 60 * 60 * 24 * 365) // 1971-01-01 00:00:00
let timeZone1 = TimeZone(secondsFromGMT: 60 * 60 * 1)! // Berlin
let start1 = startOfDayIn(date: date, timeZone: timeZone1)
let timeZone2 = TimeZone(secondsFromGMT: 60 * 60 * -8)! // New York City
let start2 = startOfDayIn(date: date, timeZone: timeZone2)

compareDates(date1: start1, date2: start2)


(-12...12).reversed().forEach { deviation in
    let timeZone = TimeZone(secondsFromGMT: 60 * 60 * deviation)!
    let start = startOfDayIn(date: date, timeZone: timeZone)

    print(start, "| UTC\(deviation >= 0 ? "+" : "")\(deviation)")
}

func adjustedStartOfDayIn(date: Date, timeZone: TimeZone) -> Date {
    var calendar = Calendar.current
    calendar.timeZone = timeZone
    let correctDay = date.addingTimeInterval(TimeInterval(-timeZone.secondsFromGMT()))
    return calendar.startOfDay(for: correctDay)
}

let correctStart1 = adjustedStartOfDayIn(date: date, timeZone: timeZone1)
let correctStart2 = adjustedStartOfDayIn(date: date, timeZone: timeZone2)

compareDates(date1: correctStart1, date2: correctStart2)