Hi,

I modified your code very slightly, and added the ThisWorkbook module code. The easiest way to save the value was to use cell 'H6' on Sheet 1, which stores the value anyway. When the workbook opens, global variable Etime gets loaded with that value.

When the workbook closes, the file is automatically saved to save the Etime value.

Your original code (Module Sheet1), modifications in red:
Dim StopTimer           As Boolean
Public SchdTime         As Variant        'Public because ThisWorkbook needs access
Public Etime            As Date           'Public because ThisWorkbook needs access
Const OneSec            As Date = 1 / 86400#

Private Sub ResetBtn_Click()              'Added in case you want to reset the value to ZERO
  Call StopBtn_click
  Etime = 0
  [h6].Value = Format(Etime, "hh:mm:ss")
End Sub
ThisWorkbook Module code (all new):
Option Explicit

Private Sub Workbook_Open()
   'Retrieve the total Time 
   Sheet1.Etime = Sheet1.[h6].Value
End Sub

Private Sub Workbook_BeforeClose(Cancel As Boolean)
  'Save the workbook at closing in order to keep the total time
  
  'Turn off the timer scheduler
  On Error Resume Next
  Application.OnTime Sheet1.SchdTime, "Sheet1.NextTick", Schedule:=False
  On Error GoTo 0
   
  ThisWorkbook.Save
End Sub
Lewis