Time span issues - I think - for subscriptions

Question

I have some user areas that I will require a check to take place on, no matter which page the user logs into.

I have a login control on the master page of each protected area and I have a table that will contain a username and date.
the date should be entered when they register and a second date which is 12 months on from when they register.

What I need to do is this.
1, check one area and check - A, at 2 months before the subscription is due and display a friendly message, B at 1 month before the due date and do the same as a, then when the date runs out I want it to then automatically forward them to a payment page.

I have no idea how to write it or to implement it, can anyone help?

Thanks