Option Explicit
Dim TStart!, TFinish!, bStop As Boolean

Private Sub Worksheet_Activate()
    CommandButton1.Caption = "Reset"
    [A1] = Int(TFinish - TStart)
    Cycle
End Sub

Private Sub CommandButton1_Click()
    TStart = Timer: TFinish = Timer: bStop = True
    ToggleButton1 = False: ToggleButton1.Caption = "Start"
    [A1] = Int(TFinish - TStart)
End Sub

Private Sub ToggleButton1_Click()
    ToggleButton1.Caption = IIf(ToggleButton1, "Stop", "Start")
    bStop = Not ToggleButton1
    If Not bStop And ToggleButton1 Then Cycle
End Sub

Private Sub Cycle()
    Do While ActiveSheet.Name = Me.Name
       If bStop Or Not ToggleButton1 Then Exit Do
       If Timer = Int(Timer) Then TFinish = Timer: [A1] = Int(TFinish - TStart)
       DoEvents
    Loop
End Sub