Calculating time difference between PM and AM

Hi guys,
I would really appreciate if someone could help me with a problem I am having.

Start time is in column D
Finish time is in column E
Rest time in column G.

I need to calculate in G the total REST hours between E6 and D7, (onwards), but the start time is PM, and the finish time is AM.

Many thanks in advance, and if anyone could recommend a TIME related tips/tutorial formula link, again I would greatly appreciate your help.

Regards
Paul