multiple validation rules

No it wouldn't I'm afraid but to go back one step:

I am only entering times, except when I have an employee that is working past midnight.

You can use MOD to calculate the hours using just time assuming no shift ever exceeds 24 hours in length, eg:

E2: =MOD(D2-C2,1)
format to h:mm

will correctly calculate the hours even if shift crosses midnight.. and so using this approach you need only enter time values into both C & D cells (6:00 pm / 12:15 am)