code to run only once and never again

[geradmcginty]

Forgive me in advance, but I have a word.dot that inserts a unique receipt number sequentially into the header of the document on open, trouble is when ever someone else opens the doc it runs again and overwrites the originally created number with a new one, I’ll need the syntax version, I thought of using the system date and time but the following is as far as I got

 
Private Sub Document_Open()  
    Dim FRun As String
    Dim LRun As String
        FRun = Now
     LRun = Now
            If LRun = FRun Then
        ‘run my code
            MsgBox (" Times are equal ") & FRun & LRun
            'run some code
           End If
        ‘don’t run code
        MsgBox (" Times are not equal ") & FRun & LRun
End Sub