Office challenge: Why does Excel change dates when I copy a sheet to a new workbook?

This week, test your Excel skills by identifying and remedying this troublesome date behavior.

Copying or moving a sheet to a new workbook is a simple task, usually. But, suppose a simple copy task ends with a call to you for help because the new workbook shifts each date value by four years and a day! For instance, the new workbook converts the original date 5/2/2012 to 5/1/2008. What's happening and can you find a happy ending for this user?