Iphone date format - ios

Question

I was trying to format a time from GMT+7 to GMT+3:
I am building an app with a world clock in specific country (the user will be at the GMT+7and I want to represent the GMT+3 time )
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setLocale:[NSLocale currentLocale];
NSDate *date = [NSDate dateWithTimeIntervalSinceReferenceDate:118800];
NSLocale *USLocale = [[NSLocale alloc] initWithLocaleIdentifier:#"en_US"];
[dateFormatter setLocale:USLocale];
NSLog(#"Date for locale %#: %#",
[[dateFormatter locale] localeIdentifier], [dateFormatter stringFromDate:date]);
I looked deep into NSDate class reference but I didn't understand how to make it.
Please if someone can help me I will be grateful.

Answer 1

There is 2 important parameters that works separately: Time and Time Zone.
e.g: Vietnam uses GMT+7
If I know that the time in Vietnam is 9:00 AM, then GMT time is 2:00 AM.
When you get the Date from your device you are getting Time and Time Zone: YYYY-MM-DD HH:MM:SS ±HHMM. Where ±HHMM is a time zone offset in hours and minutes from GMT.
Usually you are only using time. However with NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:#"GMT"] you can tell the NSDateFormatter that you want the GMT time related to your local Time Zone. So, with:
NSDateFormatter *dt = [[NSDateFormatter alloc] init];
NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:#"GMT"];
[dt setTimeZone:timeZone];
You can get the GMT date of your local time zone date.
So, If you have GMT+7: 9:00 AM and you want to print out GMT+3: 5:00 AM, you have 3 possibilities:
NSDate *localDate = [NSDate date];
OPTION 1
Add a time interval of -4 hours:
NSTimeInterval secondsInFourHours = -4 * 60 * 60;
NSDate *dateThreeHoursAhead = [localDate dateByAddingTimeInterval:secondsInFourHours];
NSDateFormatter *dt = [[NSDateFormatter alloc] init];
[dt setDateFormat:#"h:mm a"];
NSLog(#"GMT+7(-4) = %#", [dt stringFromDate:dateThreeHoursAhead]);
This is the easiest way to do it. If you are always at GMT+7 and you need GMT+3, this is a time interval of -4 hours.
OPTION 2
Set the time to GMT time zone and then add a +3hours time interval. The easiest way to do it is to add the 3 hours first and then move the time to GMT:
NSTimeInterval secondsInThreeHours = 3 * 60 * 60;
NSDate *dateThreeHoursAhead = [localDate dateByAddingTimeInterval:secondsInThreeHours];
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:#"GMT"];
[dateFormatter setTimeZone:timeZone];
[dateFormatter setDateFormat:#"h:mm a"];
NSString *date = [dateFormatter stringFromDate:dateThreeHoursAhead];
NSLog(#"GMT+3 = %#", date);
OPTION 3
This is the better option. GMT+3 is EAT (East Africa Time) you can set your time zone to EAT with: [NSTimeZone timeZoneWithName:#"EAT"]
NSDateFormatter *dt = [[NSDateFormatter alloc] init];
[dt setDateFormat:#"h:mm a"];
NSTimeZone *timeZone = [NSTimeZone timeZoneWithName:#"EAT"];
[dt setTimeZone:timeZone];
NSLog(#"EAT = %#", [dt stringFromDate:localDate]);
Option 3 is always retrieving GMT+3
An example code here.