Please Help - Capacity/Volume Question

I am trying to create a spreadsheet that shows how many people are in a department at any given time.

What I currently have;

Column A - Checkin Time (ex. 1/1/2013 00:25)
Column B - Checkout Time (ex. 1/1/2013 02:25)

Across the top I have minutes (00:00.........23:59)
Down the side I have date (1/1/2013........12/31/2013)

What I want in the cells is a formula to calculate how many people are in the department at any given time;

Ex. there would be 0 people at 00:00, 0 people at 00:01........................, 1 person at 00:25..........., etc.

At 02:25 the number of people would decrease by 1 given that the person checked out.

Any help/insight would be greatly appreciated.