获取需要增加或减少的年、月、日即可获得新的日期，上述的表示获取mydate日期前一个月的日期

- (NSDate *)getLaterDateFromDate:(NSDate *)date withYear:(NSInteger)year month:(NSInteger)month day:(NSInteger)day {
        NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
    
        NSDateComponents *comps = nil;
    
        comps = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:date];
    
        NSDateComponents *adcomps = [[NSDateComponents alloc] init];
    
         [adcomps setYear:year];
    
         [adcomps setMonth:month];
    
         [adcomps setDay:day];
    
         NSDate *newdate = [calendar dateByAddingComponents:adcomps toDate:date options:0];
    
         return newdate;
}