I have a spreadsheet that is populated with the following:

Column A: authorised hours (formatted as [h]:mm:ss)
Column B: Actual Hours (formatted as [h]:mm:ss)
Column C: Variance (Column B - Column A, formatted aS [H]:mm:ss)

If column B is greater than column A then the result is correct, but if
column A is greater than column B the result is #########. I am wanting it
to result a negative figure. For example:

A B C
10:00:00 7:00:00 -3:00:00

Can this be achieved?

Thanks