Dim blnClockRunning As Boolean

Sub startclock() 'assign start button
 blnClockRunning = True
 clock
End Sub

Sub clock()
 If blnClockRunning = False Then Exit Sub
 ActiveWorkbook.Worksheets(1).Cells(1, 1).Value = Format(Now, "hh:mm:ss")
 Application.OnTime (Now + TimeSerial(0, 0, 1)), "Sheet1.clock"
End Sub

Sub stopclock() 'assign stop button
 blnClockRunning = False
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    If Target.Address(True, True, xlA1) = "$B$1" Then
        Range("B1") = "Start"
        Range("B1").Select
        Selection.Font.FontStyle = "Bold"
        Selection.Font.ColorIndex = 2 ' White
        Selection.Interior.ColorIndex = 1 ' Black
        Call startclock
    End If
    If Target.Address(True, True, xlA1) = "$C$1" Then
        Range("C1") = "Stop"
        Range("C1").Select
        Selection.Font.FontStyle = "Bold"
        Selection.Font.ColorIndex = 2 ' White
        Selection.Interior.ColorIndex = 1 ' Black
        Call stopclock
    End If
End Sub