Calculate time in adobe form

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Javascript

I am working on an adobe form that we capture the start and end time, as well as breaks. I am looking for a java script that I can take the end time number and subtract the start time number then minus out the breaks and calculate hours worked. For instance, Start 07:00 end time 19:00 1 break (this would be an hour. .5 would be a half hour) so ((19:00-7:00)-1)=11.

Has anyone done this before?

What I have tried:

I have tried to work with the simple format features in adobe, however it will only add or multiple. No subtraction. or at least if it does I can not figure it out.