Conditional Formatting with Dates

I am trying to fill cells in a column which contain a date that has already passed. I go to 'Conditional Formatting', choose 'Manage Rules' and click 'New Rule'. I ask to input a formula, which is:

=$F<TODAY()

I choose my cell fill and hit okay, then am told by Excel, "When you use an = Excel thinks you are entering a formula."

I have seen this formula on numerous sites regarding conditional formatting and dates, and have copied and pasted the text from those websites into the formula bar, only to be told that it is not a valid formula. Am I losing my mind?