iOS单独对年月日加减

1、当前时间减去18年

+ (NSDate *)dateWithYearsBeforeNow:(NSInteger)year{    NSDate *currentDate = currentDate = [NSDate date];    NSCalendar *calendar = nil;    if (IOS8Higher) {        calendar = [NSCalendar calendarWithIdentifier:NSCalendarIdentifierGregorian];    } else {        calendar = [NSCalendar currentCalendar];    }    NSDateComponents *dateComponents = [calendar components:NSYearCalendarUnit fromDate:currentDate];    [dateComponents setYear:year * -1];    return [calendar dateByAddingComponents:dateComponents toDate:currentDate options:0];}