如何在Swift上基于GMT获得时差

Question

I am trying to get a time difference based on a GMT time. where at the end of everyday the timer resets to zero. I've tried the below code on the Xcode simulator and every time i change the time on the mac, the difference also changes.

how can i go about keeping the time to midnight set to a time zone, where the difference in timer is always the same, even if the time zone changes.

func updateTimeGMT(){

    let date = NSDate()
    var formatter = NSDateFormatter()
    var addDay = date.dateByAddingTimeInterval(60*60*24*2)

    formatter.timeZone = NSTimeZone(abbreviation: "GMT+5:30")
    formatter.dateStyle = NSDateFormatterStyle.LongStyle
    formatter.timeStyle = NSDateFormatterStyle.LongStyle

    let localDate = formatter.stringFromDate(addDay)
    let datefrom = formatter.dateFromString(localDate)

    let currentDate = formatter.stringFromDate(date)
    let currentDateFrom = formatter.dateFromString(currentDate)

    let cal = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
    let startOfToday = NSCalendar.currentCalendar().startOfDayForDate(datefrom!)

    let newDate = cal!.startOfDayForDate(addDay)

    var currentTime = currentDateFrom?.timeIntervalSinceReferenceDate
    var elapsedTime : NSTimeInterval = newDate.timeIntervalSinceReferenceDate - currentTime!

    println("\(newDate) elapsed : \(currentDateFrom)")

    let hours = UInt(elapsedTime/60/60)
    elapsedTime -= (NSTimeInterval(hours)*60*60)
    let minutes = UInt(elapsedTime/60.0)
    elapsedTime -= (NSTimeInterval(minutes)*60)
    let seconds = UInt(elapsedTime)
    elapsedTime -= NSTimeInterval(seconds)
    let fraction = UInt(elapsedTime * 100)

    let strHours = String(format: "%02d", hours)
    let strMinutes = String(format: "%02d", minutes)
    let strSeconds = String(format: "%02d", seconds)
    let strFraction = String(format: "%02d", fraction)

    countingLabel.text = "\(strHours) : \(strMinutes) : \(strSeconds) : \(strFraction)"

}

Answer 1

I think your problem is that you are using a calendar with an unset time zone.

Your calculation of adding 60*60*24*2 to the current time does not account for the two days when some timezones change to and from daylight savings time. Those days are 23 and 25 hours long.

let calendar = NSCalendar.autoupdatingCurrentCalendar()
let now = NSDate()
var startOfDay = NSDate?()
var lengthOfDaySeconds : NSTimeInterval = 0
if (calendar.rangeOfUnit(NSCalendarUnit.Day,
                         startDate:&startOfDay, 
                         interval:&lengthOfDaySeconds,
                         forDate: now)) {
    let endOfDay = startOfDay?.dateByAddingTimeInterval(lengthOfDaySeconds)
    if let secondsUntilEndOfDay = endOfDay?.timeIntervalSinceDate(now) {
        var timeLeft = secondsUntilEndOfDay
        let hours = floor(timeLeft / 3600.0)
        timeLeft -= (hours * 3600.0)
        let minutes = floor(timeLeft / 60.0)
        timeLeft -= (minutes * 60.0)
        let seconds = floor(timeLeft)
        timeLeft -= seconds
        let fraction = floor(timeLeft * 100)

        let strHours = String(format: "%02d", UInt(hours))
        let strMinutes = String(format: "%02d", UInt(minutes))
        let strSeconds = String(format: "%02d", UInt(seconds))
        let strFraction = String(format: "%02d", UInt(fraction))

        countingLabel.text = "\(strHours) : \(strMinutes) : \(strSeconds) : \(strFraction)"
    }
}