How to convert time to the time zone of the iPhone

Question

I have a time in EST timezone, it is done using the NOW() function on the mysql server. Because my server is located in EST, the time stored is in EST. When I retrieve it from my app on the iPhone, I need to display it in the correct time zone of the user. How do I do that?

Answer 1

//You can use the below function to convert date to a time zone you wish

+ (NSDate *) convertDate:(NSDate *) date toTimeZone:(NSString *) timeZoneAbbreviation {

    NSTimeZone *systemZone  = [NSTimeZone systemTimeZone];
    NSTimeZone *zoneUTC     = [NSTimeZone timeZoneWithAbbreviation:timeZoneAbbreviation];
    NSTimeInterval s        = [zoneUTC secondsFromGMT];

    NSTimeZone *myZone      = [NSTimeZone timeZoneWithAbbreviation:[systemZone abbreviationForDate:date]];
    NSTimeInterval p        = [myZone secondsFromGMT];

    NSTimeInterval i = s-p;
    NSDate *d = [NSDate dateWithTimeInterval:i sinceDate:date];

    return d;

}


//Test case **note** cgUtil is the class this method is written thus change it accordingly

__block NSDate *now = [NSDate date];
NSLog(@"Current time:%@", now);
[[[NSTimeZone abbreviationDictionary] allKeys] enumerateObjectsUsingBlock:^(NSString * abb, NSUInteger idx, BOOL *stop) {
    NSLog(@"Time zone abb:%@:Time:%@",abb,[cgUtil convertDate:now toTimeZone:abb]);
}];

Answer 2

I think it depends on what you mean by EST - if you mean East Coast US, then in general, that is 5 hours behind UTC (but not accounting for daylight saving), which should give you 04:00 EST. Try to avoid using abbreviations where possible, as they are ambiguous, e.g. EST is the abbreviation for both America/Detroit and Australia/Sydney. Using NSTimeZone initWithName will give more precise results.

The Chronos Time Zone Repository provides a nicely readable XML timezone database that really helps in understanding how time zones work (it's all rather messy and changeable).