NSDateFormatter and default locale cause bug

Question

Looks like locale setter has some magic, which forces the dateFormatter works properly

+(NSDate*)dateTimeFromJSONDateString:(NSString*)dateString{ //dateString i.e. 2013-11-19T12:47:38+04:00
    static NSDateFormatter *kDateFormatter;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        kDateFormatter = [[NSDateFormatter alloc] init];
//      kDateFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"ru_RU"];
        [kDateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZZZZ"];
    });
    NSLog(@"%@",  kDateFormatter.locale.localeIdentifier); // this one prints 'ru_RU' always either I set kDateFormatter.locale explicitly or not

    return [kDateFormatter dateFromString:dateString];
}

so this method returns nil when 6th line is commented, otherwise it returns correct date. Am I doing something wrong, or we have sdk bug here?

Answer 1

The problem solved by setting the formatter's locale to en_US_POSIX. It is necessary while parsing fixed-format date strings because the format of the date strings is constant unlike device' locale.

The code should be like

+(NSDate*)dateTimeFromJSONDateString:(NSString*)dateString{
    static NSDateFormatter *_dateFormatter;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _dateFormatter = [[NSDateFormatter alloc] init];
        _dateFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
        [_dateFormatter setDateFormat:@"yyyy-MM-dd'T'HH:mm:ssZZZZ"];
    });
    if (string)
        return [_dateFormatter dateFromString:string];
    else
        return nil;
}

Answer 2

I don't know if it's documented or is a bug, but in iOS 7 (or XCode 5, not sure of the origin of the problem) you have to set the explicitly locale of NSDateFormatter before parsing date strings, or you get nil dates

Until XCode 5/iOS 7, NSDateFormatter had as default the locale of the phone, and you didn't get a nil date

I found this problem mantaining old applications, that suddenly started returning nil dates