У меня была такая же проблема;используйте расширение для NSDate:
- (id)dateByAddingYears:(NSUInteger)years
months:(NSUInteger)months
days:(NSUInteger)days
hours:(NSUInteger)hours
minutes:(NSUInteger)minutes
seconds:(NSUInteger)seconds
{
NSDateComponents * delta = [[[NSDateComponents alloc] init] autorelease];
NSCalendar * gregorian = [[[NSCalendar alloc]
initWithCalendarIdentifier:NSCalendarIdentifierGregorian] autorelease];
[delta setYear:years];
[delta setMonth:months];
[delta setDay:days];
[delta setHour:hours];
[delta setMinute:minutes];
[delta setSecond:seconds];
return [gregorian dateByAddingComponents:delta toDate:self options:0];
}