How to create a count down formula

Hello all

I'd be grateful if anyone could advise on a formula to highlight a cell which would change colour 30 days before the expiry date. I believe this would be a conditional formatting.
Say I have a date in Cell A1, which is the expiry date of a training certificate eg 07/3/2010, I would like the cell to change colour to red 30 days before expiry to remind me to book refresher training.

If the cell could count down in months also, that would be a bonus but not essential.

Many thanks for any help offered.