Date/Time parsing in iOS: how to deal (or not deal) with timezones?

Question

I have an ASP.NET MVC 3 website that communicates with my iOS app via JSON. As part of the objects sent in the JSON response, I have dates in the format of yyyy-MM-dd HH:mm:ss ZZZ which outputs 2011-04-05 16:28:22 -07:00. How do I parse that in iOS?

This is the code I'm messing around with right now:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZZZ"];
[dateFormatter setTimeZone:[NSTimeZone localTimeZone]];
NSDate *date = [dateFormatter dateFromString:@"2011-04-05T16:28:22-0700"];

NSLog(@"%@; %@; %@", dateFormatter, date, [NSTimeZone localTimeZone]);

First thing to note is that 2011-04-05 16:28:22 -07:00 has to look like 2011-04-05T16:28:22-0700, where a T replaces the first space (assuming that stands for time, or where the time part of the string starts from?), the second space is removed and the colon in the time zone is removed. I figure I'll find a way to format the string that .NET is sending back to conform to the string iOS will parse.

The real issue is that the date that is outputted is 7 hours ahead of what I've sent in the JSON response. So, iOS outputs 2011-04-05 16:28:22 -07:00 as 2011-04-05 23:28:22 +0000, and that is wrong as far as my app is concerned.

The only solution I've found so far is to send the date in the JSON as 2011-04-05 16:28:22 +00:00, but that again is wrong because I'm altering what the real date should be.

Anyway, I'd appreciate someone taking a look and letting me know how I can parse the date string .NET is outputting via the format yyyy-MM-dd HH:mm:ss ZZZ (which I suppose can be re-written to yyyy-MM-ddTHH:mm:ssZZZ) to an NSDate object I can use in iOS.

Answer 1

I don't know how right this is, but I ultimately found that in .NET I have to do DateTime.ToUniversalTime().ToString("yyyy-MM-ddHH:mm:ss") and on the iOS side I have to do this:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setTimeZone:[NSTimeZone timeZoneWithName:@"UTC"]];
[dateFormatter setDateFormat:@"yyyy-MM-ddHH:mm:ss"];
NSDate *date = [dateFormatter dateFromString:@"2011-04-0600:28:27"];

Only then is the date correct when NSLog outputs, so I'm assuming that I've finally got a proper date/time.

Answer 2

Your iOS date parsing code is correct: 2011-04-05 16:28:22 -07:00 and 2011-04-05 23:28:22 +0000 represent the same time, just in different time zones. Your only problem is that NSDate doesn't actually store the time zone, and so [date description] outputs using UTC.

Answer 3

You're almost certainly better off just using UTC time everywhere for interchange and not bothering with time zones. I'm sure you can get that your ASP.NET code to do that, and it's easier to parse on the receiving end as well. NSDateFormatter uses standard Unicode date formatting syntax, which you can read about here.