elpi
I certainly understand what you are asking; however, Allen isn't here to help explain how his calendar works nor is it quite fair to ask us to reverse engineer his work; however, I have read his site and the explanations given are quite clear as to how the calendar works.
From that, what I would do is create a table to act as a history log that has a relationship to the employee and the project. Each time a employee is assigned a project, record the date, the employee, and the project.
You will need to review Mr. Browne's interface to determine how to do create the entries and if you run into a snag, then post the particulars for a specific issue and we'll do our best for you.