Public Class MainForm
    Dim startTime As DateTime

    Private Sub MainForm_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
        If Button1.Text = "Start" Then
            startTime = DateTime.Now
            TimerLoad.Start()
            Button1.Text = "Stop"
        Else
            TimerLoad.Stop()
            Button1.Text = "Start"
        End If
    End Sub

    Private Sub TimerLoad_Tick(sender As System.Object, e As System.EventArgs) Handles TimerLoad.Tick
        Dim timeDifferance As TimeSpan = DateTime.Now.Subtract(startTime)
        Dim newDate As DateTime = timeDifferance.ToString
        Label1.Text = newDate.ToString("HH:mm:ss.fff")
    End Sub

    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
        ListBox1.Items.Add(Label1.Text)
        If CheckBox1.Checked = True Then startTime = DateTime.Now
    End Sub

End Class