VB .NET - Contator Regressivo

Public Class Form1

    Private hora As Integer = 0

    Private minuto As Integer = 0

    Private segundo As Integer = 0

    Private milisegundo As Integer = 0

    Sub mostrarTiempo()

        Label2.Text = hora.ToString.PadLeft(2, "0") & ":"

        Label2.Text &= minuto.ToString.PadLeft(2, "0") & ":"

        Label2.Text &= segundo.ToString.PadLeft(2, "0") & ":"

        Label2.Text &= milisegundo.ToString.PadLeft(1, "0")

        Label1.Refresh()

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim TimeInicio As DateTime = "00:15:00"

        Label1.Text = TimeInicio.ToString("mm:ss")

        Timer1.Interval = 1000

        Timer1.Enabled = True

    End Sub

    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick

        Try

            Dim NovaData As DateTime = DateTime.Parse("00:" & Label1.Text)

            Label1.Text = NovaData.AddSeconds(-1).ToString("mm:ss")

            If Label1.Text = "00:00" Then

                Timer1.Enabled = False

            End If

        Catch ex As Exception

        End Try

    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click

        Timer2.Enabled = True

    End Sub

    Private Sub Timer2_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer2.Tick

        milisegundo += 1

        If milisegundo = 9 Then

            milisegundo = 0

            segundo += 1

            If segundo = 59 Then

                segundo = 0

                minuto += 1

                If minuto = 59 Then

                    minuto = 0

                    hora += 1

                End If

            End If

        End If

        mostrarTiempo()

    End Sub

    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click

        Timer2.Enabled = False

    End Sub

    Private Sub Button4_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button4.Click

        hora = 0

        minuto = 0

        segundo = 0

        milisegundo = 0

        mostrarTiempo()

    End Sub

End Class