Timer interval is changing

Imports System.IO
Imports System.Diagnostics

Public Class Form1
    Private stopwatch As Stopwatch
    Private elapsedTime As TimeSpan
    Private filePath As String = "stopwatch.txt"

    Private Sub StopwatchForm_Load(sender As Object, e As EventArgs) _
            Handles MyBase.Load
        stopwatch = New Stopwatch()

        If File.Exists(filePath) Then
            Dim storedTimeText As String = File.ReadAllText(filePath)
            Dim storedTimeTicks As Long

            If Long.TryParse(storedTimeText, storedTimeTicks) Then
                elapsedTime = New TimeSpan(storedTimeTicks)
            End If
        End If

        UpdateDisplay()
    End Sub

    Private Sub StartButton_Click(sender As Object, e As EventArgs) _
              Handles StartButton.Click
        If Not stopwatch.IsRunning Then
            stopwatch.Start()

            Timer1.Start()
        End If
    End Sub

    Private Sub StopButton_Click(sender As Object, e As EventArgs) _
                 Handles StopButton.Click
        If stopwatch.IsRunning Then
            stopwatch.Stop()
            Timer1.Stop()
        End If
    End Sub

    Private Sub ResetButton_Click(sender As Object, e As EventArgs) _
              Handles ResetButton.Click
        stopwatch.Reset()
        elapsedTime = TimeSpan.Zero
        UpdateDisplay()
    End Sub

    Private Sub Timer1_Tick(sender As Object, e As EventArgs) _
              Handles Timer1.Tick
        elapsedTime = elapsedTime.Add(stopwatch.Elapsed)
        UpdateDisplay()
    End Sub

    Private Sub UpdateDisplay()
        ElapsedTimeLabel.Text = elapsedTime.ToString("hh\:mm\:ss")
    End Sub

    Private Sub Form1_FormClosing(sender As Object, _
            e As FormClosingEventArgs) Handles MyBase.FormClosing
        If stopwatch.IsRunning Then
            stopwatch.Stop()
        End If

        File.WriteAllText(filePath, elapsedTime.Ticks.ToString())
    End Sub

End Class