Mac Numbers date functions
In Numbers 3


Select the cell (or group of cells), and from the format panel, choose Cell.
Click the conditional Highlighting and click Add a Rule...
Choose the date tab, then "after ..." from the popup. 0 as the value. This rule will appear in the popup as "date is after the date" once it is selected.
In the last box choose "Red Fill"


In Numbers '09


Select the cell (or group of cells), and from the inspector, choose Cell Format.
Under "Conditional Format" header, click the "Show Rules"
Select "With Dates", then choose "after", enter "0", choose "days", choose "from now"
Then click the edit button, click the fill square and choose the red colour.
Click "Done"