if formula to stop now() code

Open or closed what?

So if A1=Now() then B1=Closed or B1=Open?
=if(a1=now(),"Closed","open")
Placed in B1

Maybe

=IF(B1="Open",NOW(),"")

This requires VBA.
Try this

Private Sub Worksheet_Change(ByVal Target As Range)

    If Target.Count > 1 Then Exit Sub
    If Target.Column <> 11 Then Exit Sub

    If Target = "CLOSED" Then
        Target.Offset(0, -3) = Now()
    Else: Target.Offset(0, -3) = "=IF(RC[-1]<>"""",NOW(),"""")"
    End If

End Sub

And goes into the worksheet module
Where the modules are

Possibly


=IF(B1="Open",NOW(),TEXT(NOW(),"dd-mmm-yy, hh:ss"))