Laurie,
A much asked question.........
The answer is yes!!!.....Absolultely!!!!!
But it won't work.
Explanation......
The functionality to do what you want can be coded in VBA but
if the user(s) in your target audience are knowledgeable in Excel
they'll surely be able to circumvent it.
The VBA code can be locked and password protected, but a
quick search of Google will turn up plenty of threads on how
to crack the VBA password. Once the user can view the code
that you used to expire the workbook, all the efforts that expended
to do so will have proved futile.
For a good dialog (complete with examples and caveats),
take a look at this thread:
http://makeashorterlink.com/?M2D213AB6
John