arcgis desktop - Symbolizing layer with graduated colors by date/time field?

I've got a layer of polygons where each has a date associated with it. I want to color the polygons with graduated colors, but I want the colors to start with the newest polygons and fade to the oldest polygons. So far I haven't been able to find a way to do this in ArcMap (<= 9.3.1) other than to create a new field and calculate some integer or float value based on the date. Is it possible to use a date or time field directly?

• The data is natively a date-time value


• There are far too many records to use any unique-value renderer


• Converting the date to a numerical field is an answer for a different question.