Estoy tratando de convertir nsstring a nsdate y luego a systemtimezone. ¿Es correcto mi código? Cualquier ayuda apreciada.

NSString *str=@"2012-01-15 06:27:42";

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

[NSDateFormatter setDefaultFormatterBehavior:NSDateFormatterBehaviorDefault];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
NSDate *dateFromString = [dateFormatter dateFromString:str];

NSDate* sourceDate = dateFromString;

NSTimeZone* sourceTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"GMT-07:00"];
NSTimeZone* destinationTimeZone = [NSTimeZone systemTimeZone];

NSInteger sourceGMTOffset = [sourceTimeZone secondsFromGMTForDate:sourceDate];
NSInteger destinationGMTOffset = [destinationTimeZone secondsFromGMTForDate:sourceDate];
NSTimeInterval interval = destinationGMTOffset - sourceGMTOffset;

NSDate* destinationDate = [[[NSDate alloc] initWithTimeInterval:interval sinceDate:sourceDate] autorelease];

bottomLabel2.text=[NSString stringWithFormat:@"- %@ -",destinationDate];

¡El resultado del código es 2012-01-15 06:27:42 +0000 que es lo mismo que la fuente!