Excel 2007 : Conditional Formatting - Highlighting cells w/ dates older than 21 days

I'm trying to figure out how to use conditional formatting on a Column/Cell to do the following:

Example:

Cell F3 = 7/29/2011

If any cell in Column F has a Date > 21 days before TODAYS date, I want to have the cell highlighted so that the Row, or "Entry" can be further investigated. Basically it has to do with updates and how frequent each Asset is updated or checked for an update.