Use conditional formating.

The formula is assuming A1 is the cell you need formatting.

Formula is =(A1-Today())<31

Make sure you format the cell in the conditional formatting menu.

"Alexa M." wrote:

What I want to do is, when I open up excel I want a date to change colour if
it is overdue (Ie thirty-one days from the invoice date). Would I use
conditional formating or would the IF Function be better suited for this?
--
Alexa