Counting the number of days per month where where a job start time was after 10:00

I have and Excel spreadsheet that reports in column A the Date and column B the scheduled start time of each BOOKED classroom session throughout the day. The sessions have fixed start times i.e. 08:00, 10:15, 12:30, etc (each seesion is two hours with 15min gap). If asession was not BOOKED it does not appear in the spreadsheet

I have been given the task of finding out how many days each month there were no sessions BOOKED before 10:00 and after 20:15.
I could do this manually but what a pain... I am starting in Jan 2011.

How best could I achieve the task using Excel to do the work?

Thanks in advance for any help for any help