Hmmm, you might only frustrate yourself trying to do this with VBA for Excel.
If you only have one sheet in your workbook you might look at the Workbook SheetActivate event. I haven’t used it myself but perhaps you could activate your timer in there.
Private Sub Workbook_SheetActivate(ByVal Sh As Object)
//Your code here
The most thorough VB resource on the web is at MSDN, http://msdn.microsoft.com/vbasic/ but if you haven’t found an answer to your question using Google it’s unlikely to be out there on the web.