Today I was asked a fairly common question that everyone who uses calculated columns eventually runs into. “How do you get a column to calculate automatically when using a reference for a date, or something that dynamically changes like [Today]?” Out of the box this isn’t available because the calculated columns ONLY calculate when an item is updated.
The solution is simple, you write a custom solution that updates all items as desired. So this could be something that runs daily or every time you view the data (using a dashboard). You could even write the code so it adds and removes the column instead of iterating through the items. Do some performance testing to see which is faster in your case.
A terrific Article was posted by Dink that answers exactly how to do it too with free samples: http://www.sharepointblogs.com/dez/archive/2008/07/31/dynamically-updating-a-sharepoint-calculated-column-containing-a-quot-today-quot-reference.aspx
There is also a post here by Chris where he uses the DataView and JavaScript: http://pathtosharepoint.wordpress.com/2008/08/25/a-countdown-for-tasks-lists/
Hopefully this helps someone out,
Richard Harbridge
SharePoint Knowledge Base 