Subtracting Time into a Negative ??

[excel on the run]

Subtracting time with a flexible outcome & possibly a negative outcome

In the chart below:
- the "Estimated Time on Leg" is fixed
-the "Time on Leg" is variable
-the "Estimated Time Remaining" is the countdown from the "Time on Leg
-And if B4 is blank, the I want C4 to be blank


So if all is perfect it would be, C3-B4, leaving me C4 as 0:00
However if B4 is, 0:10 would leave me with, 0:01
The problem comes when B4 is 0:12, leaving me with a negative, -0:01
And when B4 is blank, then C4 needs to be blank

 

[excel on the run]

Cant seem to get the chart in the posting.
C4=C3-B4
with the exceptions above

C3 is 0:11
A4 is 0:11
B4 is 0:11
C4 is 0:00