iOS 比较时间 和 获取当前时间

// 获取系统当前的时间    NSDate *date = [NSDate date];    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];    [formatter setDateStyle:NSDateFormatterMediumStyle];    [formatter setTimeStyle:NSDateFormatterShortStyle];    [formatter setDateFormat:@"YYYY-MM-dd HH:mm:ss"];    NSString *nowTimeStr = [formatter stringFromDate:date];    NSLog(@"confromTimespStr =  %@",nowTimeStr);    //时间转为时间戳    NSString *timeStr = @"2016-07-21 17:16:29";    NSDate *datenow = [formatter dateFromString:timeStr];    NSString *timeSp = [NSString stringWithFormat:@"%ld", (long)[datenow timeIntervalSince1970]];    NSLog(@"timeSp:%@",timeSp); //时间戳的值