A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Select the entire range. A1 should be the active cell in the selection.
On the Home tab of the ribbon, click Conditional Formatting > New Rule...
Select 'Use a formula to determine which cells to format'.
Enter the formula
=AND(A1<>"", A1<=EDATE(TODAY(), -11))
Click Format...
Activate the Fill tab.
Select a fill color.
Click OK, then click OK again.