Help subtracting number entry from duration for time sheet use

Help I need to subtract break time from the contracted duration worked.
I am using a drop-down for contractors to select the time taken for a break. 10 min, 15, min etc…
I have good scripts for breaking the duration of work into {HH}:{MM}.
I just need a way to subtract the number entered above to work in a time context.
Please advise.
P.S. I am not using sheets.

10 min will be 1÷24÷60×10

Cheer Uzo I will give that a crack.