ios - 计算日期相差多少天

-(NSString*)getCurrentTimes:(int)lastDay{

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

    // ----------设置你想要的格式,hh与HH的区别:分别表示12小时制,24小时制

    [formatter setDateFormat:@"YYYY-MM-dd"];

    //现在时间,你可以输出来看下是什么格式

    NSDate *datenow = [NSDate date];
    datenow = [datenow modifyDay:-lastDay];

    //----------将nsdate按formatter格式转成nsstring

    NSString *currentTimeString = [formatter stringFromDate:datenow];


    return currentTimeString;

}

//延长或减少天数
-(NSDate *)modifyDay:(int)day{
    return [self modifyYear:0 month:0 day:day];
}

写个NSDate 分类

//延长或减少年月日
-(NSDate *)modifyYear:(int)year month:(int)month day:(int)day{
    NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    [calendar setLocale:[NSLocale currentLocale]];
    
    NSDateComponents *comps = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit) fromDate:self];
    comps.day = comps.day + day;
    comps.month = comps.month + month;
    comps.year = comps.year + year;
    return [calendar dateFromComponents:comps];
}