Displaying Negative Time

I have used a formula to subtract two dates and times. The fields are formatted in a Custom view (mm/dd/yyyy h:mm). When I subtract these two, I am trying to get the difference to display in terms of hours and minutes (hh:mm). The problem is when I have a negative difference. It displays pound signs (#) across the cell. I tried changing the formatting but none show the difference in terms of hours and minutes (it changes to decimals). I tried creating a field and using the absolute value of the times which does display the number but, since all of them are positive, it does not clearly show which ones actually had negative outcomes. As a last ditch effort, I tried going into Excel Options and changing the format to use the 1904 date system; the problem with this was that it changed all my dates in the sheet and not just the differences.

WHEW! Now to my question, is it even possible to display these times as negative without any pound signs (#) and not disturb the other information in the sheet?