extract NSTimeZone from string like '+-hh:mm'

Question

In my app I download some data as JSON from an API which contains an entry with local time:

"local_date" : "2015-07-08T13:18:14+02:00"

The JSON data are parsed into an NSDictionary* information:

NSDateFormatter* dateFormatter= [NSDateFormatter new];
dateFormatter.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ssZ";
NSDate *date = [self.dateFormatter dateFromString:information[@"local_date"]];

The date is now correctly set in UTC (which i need for some computations) but to display the local data in the user interface i need to display the date in the correct time zone.

My Question ist: How can i extract the NSTimeZone from my string? I am looking for something like NSTimeZone* timeZone = [NSTimeZone parseFromString:@"+02:00"];

I've read the documentation of NSDateFormatter, NSCalendar and NSTimeZone but didn't find out how to get the timezone from a string like mine.

Thanks in advance!

right, the data are weather data, for example for germany with +02:00. if i switch to london, the local date is +01:00. — donmarkusi
yes i want to compute with UTC but display in local time, so it is seperate. i'm gonna try parsing the string, thanks. — donmarkusi
In your hypothetical parseFromString method, what answer would you consider to be correct? — Tom Harrington
correct would be one or more timezone objects which are UTC+02:00 — donmarkusi

Tommy Tommy · Accepted Answer · 2015-07-08T12:33:36

Again: when you say "in local time", you mean local to the returned string, not to the user? So I get sent a date from Germany, I have enough information both to turn it into an NSDate and so that I could create an NSDateFormatter that recreated exactly the original string were that the output I wanted?

First of all: the time zone isn't actually in the string you supplied. It says +02:00 which tells you the offset from GMT. It doesn't tell you the time zone. E.g. an offset of +02:00 right now could be SAST, the South African time zone, could be EET, the Eastern European time zone, could be CAT, the Central African time zone, etc. There's therefore no direct way to map from offset to zone.

Supposing you just want to preserve the offset so that you can apply it later, luk2302's suggestion is probably in the right direction but I'd go the opposite way around (also I'd pay attention to QA1480):

NSString *time = @"2015-07-08T13:18:14+02:00"; // or whatever

NSDateFormatter *dateFormatter= [NSDateFormatter new];
dateFormatter.locale = [NSLocale localeWithLocaleIdentifier:@"en_US_POSIX"];

dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ssZ";
NSDate *correctDate = [dateFormatter dateFromString:time];

NSDate *dateWithoutOffset = correctDate;
NSRange rangeOfPlus = [time rangeOfString:@"+"];
if(rangeOfPlus.location != NSNotFound)
{
    dateFormatter.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
    dateFormatter.dateFormat = @"yyyy-MM-dd'T'HH:mm:ss";
    dateWithoutOffset = [dateFormatter dateFromString:[time substringToIndex:rangeOfPls.location]];     
}

NSLog(@"Offset in date was %0.0f seconds", [dateWithoutOffset timeIntervalSinceDate:correctDate]);

Because you're doing something obtuse and unusual — normal apps deal only with fixed date format strings, with dates without time zones (as within NSDates), and with displaying things in the user's local time — you'll probably need to store and deal with the offsets manually.

extract NSTimeZone from string like '+-hh:mm'

1 Answers