iOS日期的加减

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

    NSDateComponents *comps = nil;

    NSDate*nowDate = [NSDate  dateWithString:self.timeStamp formatterString:@"yyyy-MM"];

    comps = [calendar components:NSYearCalendarUnit|NSMonthCalendarUnit|NSDayCalendarUnit fromDate:nowDate];

    NSDateComponents *adcomps = [[NSDateComponents alloc] init];

    [adcomps setMonth:-];

    NSDate *newdate = [calendar dateByAddingComponents:adcomps toDate:nowDate options:];