I have seen a previous post from earlier this month on a similar subject
which didn't really answer my question.

I have a spreadsheet which i am using as a new overtime form for my office.
Staff will enter their hours worked over their daily contracted hours, and it
adds it up. however some staff take the extra time off in lieu, for example
on the following day and i would like for them to be able to enter 3:00 on
one day and -1:00 the next (for example) to give the total 2:00.

the custom format i currently have for the date (which i borrowed from
another spreadsheet we use) is [h]:mm_:_0_0

Is it possible to create an entry for -hh:mm without having to add an extra
box in which negative hours are entered to be deducted at the end?