Cell Formatting Negative Zero

[RTH10260]

Hello!
I am just adapting simple work hours collection sheet.
The row has 3 pairs of start/stop times, one cell for break, one cell for correction, all defined as Date/Time.
I make a calculation into workhours [K19] =(D19-C19)+(F19-E19)+(H19-G19)-I19+J19
Then calculate potential overtime as [L19] =IF(K19=0;"";K19-($G$8*$G$9))

Problem:
using the cell custom format of {[h]:mm;[Red]"-"[h]:mm;""} on the overtime [L19] I get a red negative zero rather than an empty field displayed.

Any suggestions as how to get arround the obvious rounding issue in the Date/Time calculation?