Reminder to pay in 90 days

I am using Excel 2003 with XP to run a home budget.  I would like to come up with a cell formula that would insert the amount to be paid in a cell if the payment is due only once every 90 days (or some other increment, such as every 6 months).  It is easy to program a payment that is due every month, or once a year, but I can't seem to come up with something that will consistently enter a payment that is due for some other time period.

 

For example, I pay my home alarm bill once every 3 months.  On the month when it is due I would like it to show up in the spreadsheet.  But for the monhts when it is not due, I would like the cell to remain blank.

 

Any suggestions?