Excel Chart

I have been charting two columns of data –‘value’ and ‘date’ to a line chart.When I move the cursor to the line it shows ‘series ‘value’ point (date) and and the ‘value’ at that point. Recently the date(ie ‘9/4′) does not appear-instead it is replaced with a five digit number(ie’39329’).I use Excel 2000
How can I retrive the ‘date’??