I have in the past used the below function to add on a specific time interval using NSDateComponents to an existing date.
(NSDate *)dateByAddin
Update: As Zaph said in his answer, Apple actually recommends using weekOfYear or weekOfMonth instead of the answer I provided. View Zaph's answer for details.
You'll probably quickly realize that you're overthinking it, but here's how you can add a certain number of weeks to a date even though the week value's been deprecated, ex:
NSDateComponents *comp = [NSDateComponents new];
int numberOfDaysInAWeek = 7;
int weeks = 3; // <-- this example adds 3 weeks
comp.day = weeks * numberOfDaysInAWeek;
NSDate *date = [[NSCalendar currentCalendar] dateByAddingComponents:comp toDate:date options:0];