时间差计算时分秒用以倒计时

NSDate * expireDate = [NSDate dateWithString:_expireDatetime formatString:@"yyyy-MM-dd HH:mm:ss.S" timeZone:[NSTimeZone systemTimeZone]];

    //算两个时间差多少秒

    CGFloat second = [expireDate secondsFrom:[NSDate date]];

    //将秒换算

    NSString * timeStr = [NSString stringWithFormat:@"%02ld:%02ld:%02ld",(NSInteger)second/3600,(NSInteger)second/60%60,(NSInteger)second%60];