I'm missing something obvious but I can't work out what.

I'm trying to calculate the difference between two time formatted cells
(hh:mm:ss) where x - y is likely to result in a negative time (for
example -07:20:00).

Excel displays the negative time as ###. If I change the formula cell to a
general format (rather than a time format) then the number is displayed but
obviously the time formatting is lost (3 and a half hours appears as 3.5).

Is there a simple way to force Excel to display negative times?