-(void)getThreeMonthDate:(NSDate *)mydate

{

    NSLog(@"%@",mydate);

    //1.创建NSCalendar

    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];

    //2.改变NSDateComponents

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

    [adcomps setYear:0];

    [adcomps setMonth:-3];

    [adcomps setDay:0];

     //3.获取NSDate

    NSDate *newdate = [calendar dateByAddingComponents:adcomps toDate:mydate options:0];

    NSLog(@"%@",newdate);

}

获取3个月前的时间(获取某一天的时间 NSDate)