iOS开发 得到当前时间前第几年第几月第几天的日期

//得到当前的时间
    NSDate * mydate = [NSDate date];
    NSDateFormatter * dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy-MM-dd"];
    NSString *currentDateStr = [dateFormatter stringFromDate:[NSDate date]];
    NSLog(@"---当前的时间的字符串 =%@",currentDateStr);
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    NSDateComponents *comps = nil;
    comps = [calendar components:NSCalendarUnitYear|NSCalendarUnitMonth|NSCalendarUnitMonth fromDate:mydate];
    NSDateComponents *adcomps = [[NSDateComponents alloc] init];
    [adcomps setYear:0];
    [adcomps setMonth:-2];
    [adcomps setDay:0];
    NSDate *newdate = [calendar dateByAddingComponents:adcomps toDate:mydate options:0]; NSString *beforDate = [dateFormatter stringFromDate:newdate];
    NSLog(@"---前两个月 =%@",beforDate);