Confusion when converting date from GMT+1 to UTC in Swift

Question

When trying to convert "2016-06-23 12:00:00" to a UTC Date I get "2016-06-23 10:00:00"

The first Date is in GMT+1 which I want to convert to UTC. If I'm not mistaken GMT+0 == UTC so 12:00 should be 11:00 right? But I always get 10:00. Why is that the case and how do I convert it correctly?

I both tried this in the playground and on an actual device

This is the Code I used:

class ViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.

    let datestring:String = "2016-06-23 12:00:00"

    print("1: "+datestring)

    print("2: "+convertDateToUTC(datestring))

}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

func convertDateToUTC(_ datestring:String) -> String {

    let dateForm = DateFormatter()
    dateForm.dateFormat = "yyyy-MM-dd HH:mm:ss"
    dateForm.timeZone = TimeZone(abbreviation: "GMT+1")

    print(TimeZone.current.abbreviation()!)

    let date = dateForm.date(from: datestring)

    dateForm.timeZone = TimeZone(abbreviation: "UTC")

    let date1 = dateForm.string(from: date!)

    return date1

}

}

output:

1: 2016-06-23 12:00:00
GMT+1
2: 2016-06-23 10:00:00

Martin R · Accepted Answer · 2016-12-04T20:17:04.243

9

Short answer: Replace "GMT+1" by "GMT+01".

"GMT+1" is not a valid time zone abbreviation:

 let tz = TimeZone(abbreviation: "GMT+1")
 print(tz) // nil

Therefore, in

dateForm.timeZone = TimeZone(abbreviation: "GMT+1")

you set dateForm.timeZone to nil, which means that the date string is interpreted in your default (local) timezone.

With

dateForm.timeZone = TimeZone(abbreviation: "GMT+01")

you'll get the expected result. Alternatively, create the time zone from the (numerical) GMT offset or from its identifier:

dateForm.timeZone = TimeZone(secondsFromGMT: 3600)
dateForm.timeZone = TimeZone(identifier: "GMT+0100")

Addendum (in response to your comments):

TimeZone(identifier: "GMT+0100") 
TimeZone(identifier: "Europe/Berlin")

are different time zones. The first one uses a fixed GMT offset of one hour, the second one is the time zone in a region (in this case, Germany), and differs from UTC by one or two hours, depending on whether daylight saving time is active at the specified date.

edited Dec 04 '16 at 20:17

answered Dec 04 '16 at 19:13

Martin R

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Thank you, that indeed made it work! What confuses me is that TimeZone.current.abbreviation()! gives me GMT+1 and not GMT+01... When i now put TimeZone(abbreviation: TimeZone.current.abbreviation()!) it will not work again, what should I do? – abcdefg Dec 04 '16 at 19:20
Aha. :))))))))) I think we should also be advising him to look at what the time zone _is_ rather than just making an assumption. – matt Dec 04 '16 at 19:20
3

@abcdefg It's best to never create a timezone from a timezone abbreviation. As you've seen, it doesn't always work. Plus there are duplicate abbreviations and plenty of invalid ones. Always use either the `identifier` or `secondsFromGMT` initializers to avoid these problems. – rmaddy Dec 04 '16 at 19:30
@abcdefg: I am sorry, but I don't know why `abbreviation()` returns "GMT+1" and not "GMT+01". But as rmaddy said, time zone abbreviations are highly ambiguous (some examples here: https://blogs.msdn.microsoft.com/oldnewthing/20080307-00/?p=23183). – `TimeZone(identifier: TimeZone.current.identifier)` should always give the original time zone. – Martin R Dec 04 '16 at 19:47
weirdly the current identifier gives me "Europe/Berlin" and applying this i end up getting 10:00 again instead of 11:00.. using TimeZone(secondsFromGMT: TimeZone.current.secondsFromGMT()) works – abcdefg Dec 04 '16 at 19:58
1

@abcdefg: "Europe/Berlin" and "GMT+01" are not the same time zone. "Europe/Berlin" is the time zone for Germany, and 2016-06-23 is during the daylight savings time. On that day, the difference to UTC is two hours. – In other words, converting "2016-06-23 12:00:00" from German time to UTC and converting "2016-06-23 12:00:00" from GMT+01 to UTC are two different things, and you have to decide which is appropriate for your application. – Martin R Dec 04 '16 at 20:03
Alright! Now I get it.. When changing the date to "2016-12-23 12:00:00" (in december) I do get 11:00 with the identifier for the TimeZone! Thank you guys so much I finally resolved this. I guess the TimeZones and DST's just confused me – abcdefg Dec 04 '16 at 20:08

Confusion when converting date from GMT+1 to UTC in Swift

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