Dans un module:
Dim temps
Sub majHeure()
Sheets("feuil1").[A1]=Now
temps = Now + TimeValue("00:00:1")
Application.OnTime temps, "majHeure"
End Sub
Sub auto_open()
majHeure
End Sub
Sub auto_close()
On Error Resume Next
Application.OnTime temps, Procedure:="majHeure", Schedule:=False
End Sub