have a text cell represent a value in a formula...

I am trying to add to this functional formula...

=IF(24*(IF(B7>C7,C7+1-B7,C7-B7))>5,24*(IF(B7>C7,C7+1-B7,C7-B7))-0.5,24*(IF(B7>C7,C7+1-B7,C7-B7)))

This formula calculates the total hours of two times, if the total hours are greater than 5 hours
it deducts 30 minutes for a lunch.

B7 and C7 have drop down list's with the follow information:

off
8:00 AM
8:30 AM
9:00 AM
9:30 AM
10:00 AM
10:30 AM
11:00 AM
11:30 AM
12:00 PM
12:30 PM
1:00 PM
1:30 PM
2:00 PM
2:30 PM
3:00 PM
3:30 PM
4:00 PM
4:30 PM
5:00 PM
5:30 PM
6:00 PM
6:30 PM

I want the formula to do the calculations, but if the cell's says "off" I want it to treat them as a value of "0".
I want the cell's to still say "off" but have a value of "0" to complete the formula.
How can I do this? Is it possible to use some sort of COUNTIF statement?

Maybe..

=IF(OR(B7="off",C7="off"),0,IF(24*(IF(B7>C7,C7+1-B7,C7-B7))>5,24*(IF(B7>C7,C7+1-B7,C7-B7))-0.5,24*(IF(B7>C7,C7+1-B7,C7-B7))))