Update timestamp only first time

Hello, I need to update timestamp value only first time when insert value into another cell and not in next updates.

This is the code that I'm using now:

Sub Worksheet_Change(ByVal Target As Range)
   
   If Target.Column = 3 Then
   
        Application.EnableEvents = False
        Dim strTime As String
        
        strTime = Format(Now, "HH:nn:ss") & "." & Right(Format(Timer, "#0.000"), 3)
        
        Cells(Target.Row, 20).Value = Date & " " & strTime
        
        Application.EnableEvents = True
        
   End If
     
End Sub