Cells A1:C1 are formatted as "h:mm:ss."
A1 is the time it took to complete a task the first attempt. B1 is the
time of the second attempt (preferably with a faster time). I want to
have C1 show the difference of the two attempts (B1-A1). This
calculates the difference perfectly if A1 is less than B1 (if it took
longer the 2nd time). However, Excel returns "######" if A1 is greater
than B1.

What is the best way to display a negative time difference (negative
being better in this case)?