DateFormatter date from string returns nil when iPhone Date & Time 24-Hour Time is off

Question

I am working on an app that initializes dates from strings returned from the backend. The dateString is returned using the following format: "2020-03-05T09:00:00+00:00"

The method I have to do the conversion is:

extension Date {
    static func convertDate(_ dateString: String?) -> Date? {
        guard let dateString = dateString else { return nil }

        let dateFormatter = DateFormatter()

        dateFormatter.setLocalizedDateFormatFromTemplate("yyyy-MM-dd'T'HH:mm:ssZZZZZ")
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"

        return dateFormatter.date(from: dateString)
    }

}

Everything was working fine until someone reported that if the user switches off "24-Hour Time" in settings the method above returns nil.

What am I doing wrong?

Thank you

Hi, I've attempted to run this code and I'm getting an error with the line 'dateFormatter.setLocalizedDateFormatFromTemplate(Date.dateFormatterLocale)', Specifically that dateFormatterLocale is not a member of Date. Is this something you have created elsewhere? I've attempted to change it to dateFormatter.locale but that gives another error. — Tom Bailey, Mar 05 '20 at 16:49
You need to set your dateFormatter locale to "en_US_POSIX". **When working with fixed format dates, such as RFC 3339, you set the dateFormat property to specify a format string. For most fixed formats, you should also set the locale property to a POSIX locale ("en_US_POSIX"), and set the timeZone property to UTC.** https://developer.apple.com/documentation/foundation/dateformatter — Leo Dabus, Mar 05 '20 at 16:54
Remove `dateFormatter.setLocalizedDateFormatFromTemplate`. You don't need it — Leo Dabus, Mar 05 '20 at 16:54
If you are also creating a timeStamp from date you should set your timeZone to UTC — Leo Dabus, Mar 05 '20 at 16:57
Note that you should set your DateFormatter locale before before setting other properties — Leo Dabus, Mar 05 '20 at 16:58
You can also use `ISO8601DateFormatter` instead of `DateFormatter` to parse your string. The default settings should work. — Leo Dabus, Mar 05 '20 at 17:00
**if you're working with fixed-format dates, you should first set the locale of the date formatter to something appropriate for your fixed format. In most cases the best locale to choose is "en_US_POSIX", a locale that's specifically designed to yield US English results regardless of both user and system preferences. "en_US_POSIX" is also invariant in time (if the US, at some point in the future, changes the way it formats dates, "en_US" will change to reflect the new behaviour, but "en_US_POSIX" will not),** — Leo Dabus, Mar 05 '20 at 17:06
**and between machines ("en_US_POSIX" works the same on iOS as it does on OS X, and as it it does on other platforms).** https://developer.apple.com/library/archive/qa/qa1480/_index.html — Leo Dabus, Mar 05 '20 at 17:06

trndjc · Accepted Answer · 2020-03-05T17:05:46.443

1

You're using a very standardized timestamp format, which allows you to take advantage of the ISO8601DateFormatter.

let dateString = "2020-03-05T09:00:00+00:00"

let df = ISO8601DateFormatter()
df.formatOptions = [.withInternetDateTime]

if let date = df.date(from: dateString) {
    print(date) // 2020-03-05 09:00:00 +0000
}

If a machine (like your server) is generating the timestamp then it will (should) always be in zulu time (GMT) so you don't need to do anything beyond this. You could specify a time zone but there isn't a point since the string will always zero it out for you.

df.timeZone = TimeZone(secondsFromGMT: 0)

This string represents an absolute moment in time. If you need a relative moment in time, such as the local time from the source, you'll need to identify that time zone and apply it here, which is also very straighforward.

edited Mar 05 '20 at 17:05

answered Mar 05 '20 at 17:04

trndjc

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This is definetly a duplicate question. There is too many questions on StackOverflow already regarding this topic – Leo Dabus Mar 05 '20 at 17:05
Note that ISODateFormatter it is only available for iOS 11 or later – Leo Dabus Mar 05 '20 at 17:07
As I already mentioned in comments the default settings should work. `df.formatOptions = [.withInternetDateTime]` it is not needed – Leo Dabus Mar 05 '20 at 17:08
When creating a timestamp with this method the timezone representation of resulting string will be different from the original string. It will use Z for UTC instead of +00:00. It means the same but it is not consistent – Leo Dabus Mar 05 '20 at 17:12
The question has been marked as duplicated but, ISO8601DateFormatter solves the issue straightaway. – Reimond Hill Mar 05 '20 at 17:34

DateFormatter date from string returns nil when iPhone Date & Time 24-Hour Time is off

1 Answers1