iOS单独对年月日加减

+ (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];
}