Please help: I'm not very good with formulas.

=IF(('May 3'!M17)<15,"15:00:00","(May 3'M17)")

May 3 M17 represents hours to carry over from the previous sheet/period to
this period. This could sometimes by negative.

The rule is that you can only ever carry over 15 hours into the next month.
The formula above needs tweaking as it still shows 15:00:00 even if the value
in the previous sheet is less than 15:00:00.

Im using 1904 date format and [h]:mm:ss format.