How to Update label on tick Event in web form - c#-4.0

Question

I have a web form in which i want to show label with time in decreasing fashion from 180 second to zero,i am using following code.
<div>
<asp:ScriptManager ID="SM1" runat="server"></asp:ScriptManager>
<asp:Timer ID="timer1" runat="server" Interval="1000"
OnTick="timer1_tick">
</asp:Timer>
</div>
<div>
<asp:UpdatePanel ID="updPnl" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Label ID="lblTimer" runat="server"></asp:Label>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="timer1" EventName="tick" />
</Triggers>
</asp:UpdatePanel>
</div>
and code behind
protected void timer1_tick(object sender, EventArgs e)
{
if (0 > DateTime.Compare(DateTime.Now,
DateTime.Parse(Session["TimeCount"].ToString())))
{
lblTimer.Text = "Number of Second Left: " +
((Int32)DateTime.Parse(Session["TimeCount"].
ToString()).Subtract(DateTime.Now).TotalSeconds).ToString();
}
else
{
}
}
and page load
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
Page.Header.Title = "Test";
NewRecord();
}
if (!SM1.IsInAsyncPostBack)
Session["TimeCount"] = DateTime.Now.AddSeconds(180).ToString();
}
problem is that counter refresh each time to 180 when user click on button,and label is static value like 177,173 etc,i want to show changing in counting is shown to user?it is basically Q/A software.