Proper Time format

I'm only posting here due to my previous thread, http://www.excelforum.com/showthread.php?t=594567
they are related.

The time being entered is minutes and seconds, i.e., 20:30, but excel views that as 12:20:30 PM. I've set the cell format to [mm]:ss, thinking it would then see that as minutes:seconds and all I would have to do is type in the cell, 20:30, then have excel see that as 20 minutes and 30 seconds.

It does, however, display the cell as 20:30, but when you select the cell, the display bar up top displays it as 12:20:30, which leads me to believe that excel is seeing this as reading time on a clock an not as elapsed time as I think it should be.

If i type in the cell, 00:20:30, it sees it as 12:20:30 AM

I'm somewhat bewildered by this.............i can probably let it go as hh:mm and have the calculations work, for the intent of the spreadsheet, but I just prefer everything display properly.

My other problem is that i want to be able to display time deficiency, i.e. 2 minutes 30 seconds as -02:30 .

thanks.