Timer to check time during exam

We do not do your homework: it is set for a reason. It is there so that you think about what you have been told, and try to understand it. It is also there so that your tutor can identify areas where you are weak, and focus more attention on remedial action.

Try it yourself, you may find it is not as difficult as you think!

Now this is the way I solved it with the help given me above by Karthik:

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

       Dim timer_arr() As String
       Dim timer As String
       timer = Button1.Text

       timer_arr = timer.Split(":")
       Session("seconds") = Integer.Parse(timer_arr(2))
       Dim minutes = Integer.Parse(timer_arr(1))
       Dim hours = Integer.Parse(timer_arr(0))
       If Session("seconds") = 0 And minutes = 0 And hours = 0 Then
           Timer1.Enabled = False
       Else
           Session("seconds") = Session("seconds") - 1
       End If
       If (Session("seconds") < 0) Then
           Session("seconds") = 59
           Session("minutes") = Session("minutes") - 1
       End If
       If Session("minutes") < 0 Then
           Session("Minutes") = 59
           Session("TimerRemaining") = Session("TimerRemaining") - 1
       End If

       Button1.Text = Session("TimerRemaining").ToString() & ":" & Session("Minutes").ToString & ":" & Session("Seconds").ToString

       If Session("TimerRemaining") = 0 And Session("minutes") = 0 And Session("seconds") = 0 Then
           Button1.Enabled = False
           Timer1.Enabled = False
       End If
   End Sub

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load


        If Not IsPostBack Then
            Session("TimerRemaining") = 1
            Session("Minutes") = 0
            Session("Seconds") = 0
            Button1.Text = Session("TimerRemaining").ToString() & ":" & Session("Minutes").ToString & ":" & Session("Seconds").ToString
        End If
   End Sub

<form id="form1" runat="server">

                        <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Button ID="Button1" runat="server" Text="Button" />
                <asp:Timer ID="Timer1" runat="server" Interval="1000">
                </asp:Timer>
            </ContentTemplate>
        </asp:UpdatePanel>
    </div>