Nil NSDate when trying to get date from UTC string in zulu time

Question

Writing an iPhone app in Objective-C, I have a date in string form (in UTC format, with a Z on the end to denote zero UTC offset, or zulu time), which I need to parse into an NSDate object.

A bit of code:

NSDateFormatter* df = [[NSDateFormatter alloc]init];
[df setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSZ"];
NSString* str = @"2009-08-11T06:00:00.000Z";
NSDate* date = [df dateFromString:str];

Running this through the debugger, date ends up nil! I'm assuming it has something to do with my date format string.

How can I fix it to correctly parse the date string?

A thought would be to make the Z in the date format literal, a la setting the date format to yyyy-MM-dd'T'HH:mm:ss.SSS'Z'.

That would work, except when the Z is parsed as a literal, the date loses offset information, and so is ambiguous, and therefore interpreted to be local time.

For example, if the string to parse was 2009-08-11T06:00:00.000Z (6:00 zulu time) it would be interpreted as 6:00 local time, and an incorrect offset would then be applied. It would then be parsed as 2009-08-11T06:00:00.000-600 (12:00 zulu time) with the offset depending on the user's offset.

Thanks!

Answer 1

I've had this problem also, I'm not sure if it's a API bug within Apple's code, or my lack of understanding, but I've worked around it by using hour offsets in my date strings.

If you change the code in your example to:

NSDateFormatter* df = [[NSDateFormatter alloc]init];
[df setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSZ"];
NSString* str = @"2009-08-11T06:00:00.000-0700";   // NOTE -0700 is the only change
NSDate* date = [df dateFromString:str];

It will now parse the date string. Of course the -0700 hours is my offset, you'd have to change it to yours. Hope this helps.

Answer 2

Most answers suggest you to treat 'Z' as a literal character. Do not do this!

The Z actually means that the date is offset by 0 to UTC (+0000).

This is according to the time zone format ISO8601:

ISO 8601 time zone format: A constant, specific offset from UTC, which always has the same format except UTC itself ("Z").

"-08:00"

"Z"

What you want to do is use the following format for your NSDateFormatter:

 NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
                                  // 2013-11-18T23:00:00.324Z
                                  [formatter setTimeZone:[NSTimeZone localTimeZone]];
                                  [formatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSSZZZZZ"];
                                  return formatter;

By repeating the Z five times, you tell the formatter to use ISO8601 when parsing the string.

Bonus:

• Use one to three Zs for RFC 822 GMT format.
• Use four Zs for localized GMT format.

For more information check this document.

Answer 3

I think you need to put single quotes around the Z in the format string, because the Z actually means something to the formatter and you want it to represent a literal character instead.

    [df setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"];

Answer 4

There's no need to manipulate the string. Simply set the time zone on the NSDateFormatter (and the locale while you're at it):

NSDateFormatter * formatter = [[NSDateFormatter alloc] init];
[formatter setTimeZone:[NSTimeZone timeZoneForSecondsFromGMT:0]];
[formatter setLocale:[NSLocale systemLocale]];
[formatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"];

And parse date strings as needed:

NSDate * value = [formatter dateFromString:@"2012-03-01T23:08:25.000Z"];
NSLog(@"%@", value); // prints 2012-03-01 23:08:25 +0000

Answer 5

You can use this method to get date from UTC.

+ (NSDate*)getDateFromUTCDateTimeString:(NSString*)dateString {

NSDateFormatter *isoDateFormatter = [[NSDateFormatter alloc] init];

[isoDateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"];

[isoDateFormatter setTimeZone:[NSTimeZone  timeZoneWithAbbreviation:@"UTC"]];

NSDateFormatter *userFormatter = [[NSDateFormatter alloc] init];

[userFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];

[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];

[dateFormatter setTimeZone:[NSTimeZone  timeZoneWithAbbreviation:@"UTC"]];

NSDate *date = [isoDateFormatter dateFromString:dateString];

return [dateFormatter dateFromString:[userFormatter stringFromDate:date]];

}

Answer 6

this may help you.. the "Z" is a literal for the time zone code. try using "o" (the letter, not zero). The literal "o" means UTC offset. I ran into this a while back, I hope this helped you.

Answer 7

-(NSDate*)dateFromZulu:(NSString*)str {
    if (str == nil) {
        NSLog(@"Error getting date");
        return [NSDate date];
    }

    NSDateFormatter *f = [[NSDateFormatter alloc] init];
    [f setDateFormat:@"yyyy-MM-dd'T'HH:mm:ss Z"];
    NSDate *ret = [f dateFromString:[str stringByReplacingOccurrencesOfString:@"Z" withString:@" +0000"]];
    [f release];

    if (ret == nil) {
        ret = [NSDate date];
        NSLog(@"Error formatting date (%@)",str);       
    }   
    return ret;     
}