I manage the use of multiple instances of licences and have recorded each one
on a spreadsheet complete with the last access date. This spreadsheet i
update on a monthly basis and this allows me to monitor if a licences is
being used effectively.
In the first column I have the names of the PCs using the licences. In the
next 12 columns are the last access dates for the various projects.

I would like to set each cell to change it's background depending on the
date entered. E.G. I need the background of the cell to go to red if the date
typed in is greater than three months or 90 days previous to the current date.