Excel 2007 : Timesheet accruing sick time every month issue

I am working with a timesheet and need to add a half day of AA (sick time) at the end of every month. The column is K6 and the formula assumes that the employee has 50 days of AA or sick time as of now and then subtracts the time as it is used. Is there a way to calculate a 1/2 day every month? I have attached my timesheet. Thanks