Attendance database setup

I am creating an attendance database for my company.

There are 19 different departments in the company and each manager tallies
his/her own employee attendances.

I had a spreadsheet set up, but it wasn't on a rolling calendar basis. Then,
I was going to have the managers delete old months, but that would most
likely mess with the formulas.

So the new spreadsheet needs to be on a rolling calendar basis, so that when
a new month comes up, the previous month from last year will drop off in
calculations.

My file has three worksheets, one titled "Summary", one titled "Details" and
the last "Setup".

The Summary page should tally the total absences from the year for each
employee. It serves as a quick view. The Details page is where Managers
enter information about each individual employee, each day they are late, or
don't show up for work. What I would like to do on the set-up page is to
create a reference start date, so the Details page can reference this and add
365 days...and sum the number of absences within that range. I don't know how
to do this. How should the sheets be set up. Can someone please help?

Thank you.