Run a macro when the date is changed..

Hello,

I have a simple code to continue a serial number every time i reopen a workbook,.. heres the code..

Private Sub Workbook_Open() 

Sheets("Sheet1").Range("A5") = Sheets("Sheet1").Range("A5") + 1 
End Sub

(this code was done for me by "Rizvisa)

I would like to know if its possible for this formula to work only if i change the date on cell E7.

Thanks....