Can some plaese tell me how I can get a negative time to show?

What I mean is this coloumn A as total worked time and coloumn B as weeks
hours, so what i want is the sitiation where if coloumn A is smaller than B I
get a negative time so that the rolling toil time is maintained ie week 1 I
generate 8 hours toil week 2 I generate -2 hours toil so toatl is 6. The
cells are formatted as time in the fashion of 37:30:55.

MTIA