Tracking accrued sick time with excel
 

I need to setup a worksheet that will allow me to track accrued sick time up
to a limit (72 hours). Basically for every 30 hours worked accrues 1 hour of
sick time. If possible I would also like to enter the date of hire and have
the accruals automatically start after 90 days. I would also like to be able
to subtract hours as they are used up but have the accrual continue to add up
as more hours are worked. Can anyone suggest how to begin this worksheet.
Thanks