Chart Data Changes Data Type

[AJ]

Using Excel 2007, I have a line chart in Workbook A that is built from data
in Workbook B. The X axis contains dates. When viewing the chart in Workbook
A the dates a properly formatted when I have Workbook B open. However, when
I close Workbook B the dates change to numbers. When I open Workbook B the
numbers change back to dates. How can I keep the dates in Workbook A to stay
in date format?

Thank you.

AJ