Absence Calculator

im trying to put together a system on worksheets that checks 'Absence' in a rolling 12 month period. The 12 month period is any 12 months and not a financial period (eg 25/12/06 -25/12/07).

I have 36 employees and want to have their names in each sheet, calander dates across the top, will mark either a 'S' for sick or 'L' for late ect against the dates if not at work.

Once an absence has been entered, on the sheet somewhere it will show how many days that person has been off (eg, 10th Oct, 16 Nov and 22 Dec would = 3 Days absence).

How do I set up the sheets to work out how many days each person has had off in a rolling 12 month period (so that it does not calculate beyond the 12 months).

I have looked on here to see if there are any programs, formulas which may work but some have lost me in my tracks.