Use NSCalendar to define the calendar system (we use the standard gregorian calender)

Use NSDateComponents to add or subtract components(days, months, year or time) from or to date.

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *comps = [[NSDateComponents alloc] init];

[comps setMonth:-11];                    // here setting the Month component to -11, so I am subtracting 11 months from the current date

NSDate *toDate = [gregorian dateByAddingComponents:comps toDate: [NSDate date] options:0];

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Use NSDateFormatter class to playing around with dates.

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

//use MM for 2 digit month, MMM for 3 char style and MMMM for full month name

[dtFrmt setDateFormat:@"MM/dd/yyyy"];              // like 01/31/2010

[dtFrmt setDateFormat:@"hh:mm:ss a"]                 // like 10:01:00 am

[dtFormat setDateFormat:@"EEEE hh:mm a"];    // like Sunday 11:11 pm