I'm guessing it's seeing a different activesheet when you go to a different
window.  You're going to have to specify the workbook and sheet you want, I
think.  

Try this

Option Explicit

Public nTime As Double

Public Sub StartTimer()
Dim aWB As Workbook
Dim aWS As Worksheet

Set aWB = ThisWorkbook
Set aWS = aWB.Worksheets("Sheet1")  'change sheet name as needed
   On Error Resume Next
   Application.OnTime nTime, "RunTimer", , False
   On Error GoTo 0

   aWS.Range("A1").Value = 0
   RunTimer
End Sub

Public Sub StopTimer()
   Application.OnTime nTime, "RunTimer", , False
End Sub

Public Sub RunTimer()
Dim aWB As Workbook
Dim aWS As Worksheet

Set aWB = ThisWorkbook
Set aWS = aWB.Worksheets("Sheet1")  'change sheet name as needed
   
   With aWS.Range("A1")
       .Value = .Value + TimeSerial(0, 0, 1)
       .NumberFormat = "hh:mm:ss"
   End With
   nTime = Now + TimeSerial(0, 0, 1)
   Application.OnTime nTime, "RunTimer"
End Sub

hi barb it is working ok ty !!
but if i had to insert my macro , where should i put "her" for avoiding it
to run multiply in each code cycle,  when i open many times the same window ?
ty!