Change date display after specified time

[gcol]

I have a few columns of dates and I'd like them to be displayed in a different colour (or have the cell change colour) when 6 months has passed. For example if the date in the cell is 12/09/06 and the current date is 13/03/07 then it'd be over 6 months so I'd like the date 12/09/06 to change to a red text colour or have the cell change colour. Or even better, to have it flash.