How get current time zone offset ?

Question

Hi,
I am facing to a problem related getting time zone offset.
Below code is used for my application :
NSDate* sourceDate = [NSDate date];
NSTimeZone* destinationTimeZone = [NSTimeZone systemTimeZone];
timeZoneOffset = [destinationTimeZone secondsFromGMTForDate:sourceDate] / 3600;
But sometimes seems that above code is getting work wrong. For example :
Knoxville, TN : time zone offset should be -5. But my code resulted it as -4.
http://www.timeanddate.com/worldclock/city.html?n=843
This web is related my issue. Here is :
Standard time zone: UTC/GMT -5 hours
Daylight saving time: +1 hour
Current time zone offset: UTC/GMT -4 hours
Actually I don't know how get to "Daylight saving time:".
Can you correct this issue for me ?
Please advice.
Thanks.

Answer 1

Hi Raiden -
RaidenMAC wrote:
Knoxville, TN : time zone offset should be -5. But my code resulted it as -4.
I think your code is giving you the correct offset for Knoxville on today's date. The offset for Eastern Standard Time (EST) is -5, but Knoxville is currently on Eastern Daylight Time (EDT), which is one hour later than EST. (-5) + 1 => (-4), which is what your code computed.
Note that [secondsFromGMTForDate:|http://developer.apple.com/library/ios/documentation/Co coa/Reference/Foundation/Classes/NSTimeZoneClass/Reference/Reference.html#//appleref/doc/uid/20000190-CACDIAIC] is correcting for daylight savings time. That's why you need to pass the date to that method. If you pass a "summer" date, the return offset will be one hour more than it will be for a "winter" date (usually early November to mid March). For example, try moving the date forward 60 days like this:
NSDate *sourceDate = [NSDate dateWithTimeIntervalSinceNow:3600 * 24 * 60];
NSTimeZone* destinationTimeZone = [NSTimeZone systemTimeZone];
int timeZoneOffset = [destinationTimeZone secondsFromGMTForDate:sourceDate] / 3600;
NSLog(@"sourceDate=%@ timeZoneOffset=%d", sourceDate, timeZoneOffset);
The above should give you the offset for Knoxville on Dec. 7, 2010, which is a winter date.
Actually I don't know how get to "Daylight saving time:".
You can test a NSTimeZone object with [isDaylightSavingTimeForDate:|http://developer.apple.com/library/ios/documentat ion/Cocoa/Reference/Foundation/Classes/NSTimeZoneClass/Reference/Reference.html#//appleref/doc/uid/20000190-CHDCJGHE], but you don't need that information when you obtain the offset with secondsFromGMTForDate:.
- Ray