A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Select the cell or cells with dates.
On the Home tab of the ribbon, click Conditional Formatting > New Rule...
Select 'Format only cells that contain'.
Leave the first dropdown set to 'Cell Value'.
Select 'less than' from the second dropdown.
In the box next to it, enter the formula =EDATE(TODAY(),-12)
Click Format...
Activate the Fill tab.
Select a color.
Click OK, then click OK again.