Converting date between timezones swift

问题

I have a date stored on my online server database which is in GMT. I load the date and convert it to the user's timezone using the following code :

 if let messagedate = oneitem["timestamp"] as? String {
     let dateFormatter = NSDateFormatter()
     dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
     let date = dateFormatter.dateFromString(messagedate)
     let source_timezone = NSTimeZone(abbreviation: "GMT")
     let local_timezone = NSTimeZone.systemTimeZone()
     let source_EDT_offset = source_timezone?.secondsFromGMTForDate(date!)
     let destination_EDT_offset = local_timezone.secondsFromGMTForDate(date!)
     let time_interval : NSTimeInterval = Double(destination_EDT_offset - source_EDT_offset!)


     let final_date = NSDate(timeInterval: time_interval, sinceDate: date!)
     curr_item.date = final_date
 }

Now I need to convert the date back to GMT in order to communicate it to the server, however I'm not sure how to convert it back to GMT.

回答1:

Couldn't you just use your data formatter again with a different time zone and convert it? Such as

dateFormatter.timeZone = NSTimeZone(abbreviation: "GMT")
let gmtDate = dateFormatter.dateFromString(string: "your old date as string here")

回答2:

about 50 times more effecient

extension Date {
    func convertToLocalTime(fromTimeZone timeZoneAbbreviation: String) -> Date? {
        if let timeZone = TimeZone(abbreviation: timeZoneAbbreviation) {
            let targetOffset = TimeInterval(timeZone.secondsFromGMT(for: self))
            let localOffeset = TimeInterval(TimeZone.autoupdatingCurrent.secondsFromGMT(for: self))

            return self.addingTimeInterval(targetOffset - localOffeset)
        }

        return nil
    }
}

回答3:

Simpler version:

extension Date {
    func convertToTimeZone(initTimeZone: TimeZone, timeZone: TimeZone) -> Date {
         let delta = TimeInterval(timeZone.secondsFromGMT() - initTimeZone.secondsFromGMT())
         return addingTimeInterval(delta)
    }
}

回答4:

Since NSDate is always in GMT/UTC the time zone only becomes relevant when displaying it to, or getting it from, the user. Just always assume it's UTC internally, convert it for the user (by setting it on the NSDateFormatter) as necessary, and you no longer have to worry about the problem.

回答5:

Based on mukaissi's answer, but the order of deductible in the expression has been corrected.

extension Date {    
    func convert(from initTimeZone: TimeZone, to targetTimeZone: TimeZone) -> Date {
        let delta = TimeInterval(initTimeZone.secondsFromGMT() - targetTimeZone.secondsFromGMT())
        return addingTimeInterval(delta)
    }    
}

回答6:

I suggest

• you set the GMT timezone on your dateFormatter to get back directly a NSDate in UTC (having only NSDates in UTC is a good practice)
• when you need to display it you use another NSDateFormatter with the local time zone set on it (it is by default)
• when you need to send a date to your server, you use dateFormatter again to generate a string

来源：https://stackoverflow.com/questions/38641982/converting-date-between-timezones-swift