Conditional Formatting Using If Statement On Opening Of Workbook

I have several worksheets in a workbook that has dates of completion of
various training topics. Some of these topics require annual (12 months),
bi-annual (24 month), 6-month, or even 2-month re-training.

What I am trying to accomplish is to automatically shade all cells for dates
that fall within one month of these periodicities based on the computer's
clock when a workbook is opened. Unfortunately different columns have
different periodicities so what I was planning on doing was putting that
monthly number in a cell ("12" in A1000, for example for all Column A
values). When the file is opened, if any of the dates in column A are more
than 11 months from today's date (A1000 minus 1), they are within one month
of needing to be re-trained and I want the cell shading to be a light
yellow.