scheduling problem

Hi folks;
I have been grappling cwith this for months but cant work it out.
I have about 70 sites, listed by name, address and phone number, that I have to visit either monthly, two or three monthly, or six monthly. I need to automatically calculate a list for each month based on the visiting frequency. If I miss a month, the visit comes due the following month and each visit thereafter needs to be recalculated to reflect the change, based on the visiting frequency.
I have attached an excel sheet to show you what I mean. Each month needs to be on a separate page and the 'last visit' is the determinant of which page the subsequent visits should show on.
Can anyone help?