Objective-C, How can I get the current date in UTC timezone?

Question

I am trying:

NSDate *currentDateInLocal = [NSDate date]; NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init]; [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:SS.SSS'Z'"]; NSString *currentLocalDateAsStr = [dateFormatter stringFromDate:currentDateInLocal];  NSDateFormatter * dateFormatter2 = [[NSDateFormatter alloc] init]; NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:@"UTC"]; [dateFormatter2 setTimeZone:timeZone]; [dateFormatter2 setDateFormat:@"yyyy-MM-dd'T'HH:mm:SS.SSS'Z'"]; NSDate *currentDateInUTC = [dateFormatter2 dateFromString:currentLocalDateAsStr]; 

but It's still does not represent the current UTC time, how can I achieve this?

Thanks

Answer 1

You're overcomplicating things.

NSDates don't have time zones or calendars. [NSDate date] gets the current date, which is a measurement of a moment in history. If I run [NSDate date] in Europe at exactly the same time as you run it in America then we'll get exactly the same value.

How you print a date depends on the calendar and the time zone. So a date printed in the Gregorian calendar looks different from the same one printed in the Julian calendar. And a date printed in the UTC Gregorian calendar looks different from the same one printed in the PST Gregorian calendar. But they're still the same date.

So you want to jump straight to your dateFormatter2.

Answer 2

The accepted answer by Alex Wien is incorrect.

By default, NSDateFormatter adjusts the NSDate’s date-time value from UTC to the user's local time zone. To prevent that adjustment, tell the NSDateFormatter to use the time zone for UTC.

To verify results, google "current time utc".

My source code below should do the job, meaning get the current date-time as a string in ISO 8601 format in the UTC (Zulu) time zone signified by a Z on the end.

NSDate* datetime = [NSDate date]; NSDateFormatter* dateFormatter = [[NSDateFormatter alloc] init]; [dateFormatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]]; // Prevent adjustment to user's local time zone. [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"]; NSString* dateTimeInIsoFormatForZuluTimeZone = [dateFormatter stringFromDate:datetime]; 

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You could put this logic in a pair of convenience methods somewhere in your app.

- (NSString*)now {     // Purpose: Return a string of the current date-time in UTC (Zulu) time zone in ISO 8601 format.     return [self toStringFromDateTime:[NSDate date]]; } 

…and…

- (NSString*)toStringFromDateTime:(NSDate*)datetime {     // Purpose: Return a string of the specified date-time in UTC (Zulu) time zone in ISO 8601 format.     // Example: 2013-10-25T06:59:43.431Z     NSDateFormatter* dateFormatter = [[NSDateFormatter alloc] init];     [dateFormatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];     [dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"];     NSString* dateTimeInIsoFormatForZuluTimeZone = [dateFormatter stringFromDate:datetime];     return dateTimeInIsoFormatForZuluTimeZone; } 

Example of usage…

NSString* now = [self now]; 

Or turn those minus signs into plus signs to use as class methods rather than instance methods…

NSString* now = [SomeClassNameHere now]; 

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Tip: For better readability by humans, change that T in the format to a SPACE. For better interoperability by software, keep the T. The ISO 8601 spec tolerates a space but recommends keeping the T.

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Tip: I've not tested, but… Some people say instantiating [NSDateFormatter][4] is expensive. If doing so often (such as in a loop) consider caching a single instance for re-use.