one way:
Put this in the ThisWorkbook code module (Right-click the workbook title
bar and choose View Code):
Public Sub Workbook_Open()
If Date > #12/15/2004# Then _
Sheets("Sheet1").Range("A1,B2,J10,K39").ClearContents
End Sub
Note that this won't work if the user opens your workbook with macros
disabled.
Note also that there's no way to prevent a user from accessing your data
- i.e., they can set their clock back and open the file. You can make
things more difficult, but not much...