My suggestion is to avoid using Duration field type where possible. Your life will be easier if you coax all the calculations to be numeric hours.
Sounds like you have a single record where the employee enters their hours for the days they worked that week. That in fact is a very clean way to do it from a Quickbase point of you because all the data you need us in one record.
I would suggest making five formula in numeric fields like this:
Field Name: [Day 1 Hours]
Nz(ToHours(
ToTimeStamp([Day 1 Date End ],[Day 1 Time End])
-
ToTimeStamp([Day 1 Date Start],[Day 1 Time Start])))
// the Nz says to treat a null value as zero
Then the formula for [Total Hours Worked] is just
[Day 1 Hours]+ [Day 2 Hours] + ... etc [Day 7 Hours] .
He would show that field on the form right beside where the users entering their start and end dates and times
Then you have a field for [Standard Weekly Hours] make that field be a numeric field type, and your setup might be that it's just a formula field with formula of say 40. Maybe it varies by the State that the worker is in or maybe it's a value which is locked up from the employee record. But best if its a numeric field type.
Then [OT hours] would be
Max(0, [Total Hours Worked] - [Standard Weekly Hours])
The above formula takes the hours worked minus the standard hours but if the result is smaller than zero then it's going to take the maximum (ie the higher) of zero and some negative number meaning that this can never be negative.
Then the [OT Pay] will be
[OT hours] * [OT Pay Rate]