Public Class Form1
    Dim onOff As Integer

    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        Button1.Text = Date.Now
    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Timer1.Interval = 1000
        onOff += 1
        If Not (onOff Mod 2 = 0) Then
            Timer1.Start()
        Else
            Timer1.Stop()
        End If
    End Sub
End Class