Defining core time periods in a day

Hi

Hopefully someone can help me, it would be much appreciated

Basically I have a call centre system that works on 3 "sessions" per day which are:

8.30 - 12.30
12.30 - 16.30
16.30 - 20.00

I then have 2 dates/times, a date/time a call was logged, and the date/time it was closed down:

Time logged: 25/11/2011 12:16
Time closed down: 26/11/2011 17.46

Imagine you logged a call with a helpdesk, and instead of having a fixed time to rectify the fault (ie 24hrs), you have certain number of "sessions" to fix the problem. So in the example above, it took 5 sessions to fix (including the "session" it was logged in and closed down in).

Is there anyway of defining the session times in excel, then counting how many times these sessions repeat between 2 dates. All I am interested in is getting the whole number of sessions.

Anyone have an idea if this can be done in excel?

Thank you!