iOS 计算两个时间之间的差

NSDateFormatter *date=[[NSDateFormatter alloc] init];
    [date setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
    NSCalendar *cal=[NSCalendar currentCalendar];
    unsigned int unitFlags=NSYearCalendarUnit| NSMonthCalendarUnit| NSDayCalendarUnit|NSHourCalendarUnit|NSMinuteCalendarUnit|NSSecondCalendarUnit;
     NSDateComponents *d = [cal components:unitFlags fromDate:[date dateFromString:@"2014-11-24 17:21:58"] toDate:[date dateFromString:@"2014-11-25 17:25:36"] options:0];
    NSLog(@"%d天%d小时%d分钟%d秒",[d day],[d hour],[d minute],[d second]);