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;
}
