time difference formula - need fix

Hi,

this is my formula for calculating time differences :

=IF(OR(E5=0;F5=0);0;F5-E5+(F5<E5))

formula works, but It acts very odd when calculating with midnight hours (00:00:00). I have Combobox above start/end cells, which inserts selected time interval in them, and If I select time interval with midnight, formula doesn't calculate.

But If I select midnight hour manually from drop-down lists of start/end cells, formula works.

Looks like some problem with time format. How can I fix this ?

Please take a look at the sample, allready selected midnight from combobox.

thanks for all help !