计算 某一天据今天有多少天

- (int)intervalSinceNow: (NSString *) theDate {

    NSDateFormatter *date=[[NSDateFormatter
alloc]
init];

    [date setDateFormat:@"yyyy-MM-dd"];// 传入格式
为   2017-09-18

    NSDate *d=[date
dateFromString:theDate];

    

    NSTimeInterval late=[d
timeIntervalSince1970]*1;

    NSDate* dat = [NSDate
dateWithTimeIntervalSinceNow:0];

    NSTimeInterval now=[dat
timeIntervalSince1970]*1;

    NSString *timeString=@"";

    NSTimeInterval cha=now-late;

    if (cha/86400>1) {

        timeString = [NSString
stringWithFormat:@"%f", cha/86400];

        timeString = [timeString substringToIndex:timeString.length-7];

        return [timeString
intValue];

    }

    return -1;

}