I am trying to figure out why my formula isn't working. I am creating a
timesheet form and have correctly calculated the number of hours worked based
on a start and end time.

My problem is in needing to break out overtime. Currently, I am returning a
value of 12:30 total hours for an 8:30 am to 9 pm work day. In my next cell
down, I need to indicate how many of those hours are overtime (which is
anything over 10 hours, so the correct answer is 2:30). If there is no
overtime, I want to return a value of 0. (I would also be ok with returning
no value at all if there is no overtime.)

The formula that I am using is this:
=IF(B12=10,SUM(B12-10),"0")

The formula dialog is telling me that B12 (which has a value of 12:30) is
not greater than 10 (which is not true) and therefor returns a value of 0
since the calculation concludes that there is no overtime.

What am I doing wrong?