How To Timer Work in Asp.net

Using a Timer Control Inside an UpdatePanel Control

When the Timer control is included inside an UpdatePanel control, the Timer control automatically works as a trigger for the UpdatePanel control. You can override this behavior by setting the ChildrenAsTriggers property of the UpdatePanel control to false.
For Timer controls inside an UpdatePanel control, the JavaScript timing component is re-created only when each postback finishes. Therefore, the timed interval does not start until the page returns from the postback. For instance, if the Interval property is set to 60,000 milliseconds (60 seconds) but the postback takes 3 seconds to complete, the next postback will occur 63 seconds after the previous postback.
The following example shows how to include a Timer control inside an UpdatePanel control.

<asp:ScriptManager runat="server" id="ScriptManager1" />
<asp:UpdatePanel runat="server" id="UpdatePanel1" 
    UpdateMode="Conditional">
  <contenttemplate>
    <asp:Timer id="Timer1" runat="server"
      Interval="120000" 
      OnTick="Timer1_Tick">
    </asp:Timer>
  </contenttemplate>
</asp:UpdatePanel>

Using a Timer Control Outside an UpdatePanel Control

When the Timer control is outside an UpdatePanel control, you must explicitly define the Timer control as a trigger for the UpdatePanel control to be updated.
If the Timer controls is outside an UpdatePanel control, the JavaScript timing component continues to run as the postback is being processed. For example, if the Interval property is set to 60,000 milliseconds (60 seconds) and the postback takes 3 seconds to complete, the next postback will occur 60 seconds after the previous postback. The user will see the refreshed content in the UpdatePanel control for only 57 seconds.
You must set the Interval property to a value that enables one asynchronous postback to complete before the next postback is initiated. If a new postback is initiated while an earlier postback is being processed, the first postback is canceled.
The following example shows how to use the Timer control outside an UpdatePanel control.

<asp:ScriptManager runat="server" id="ScriptManager1" />
<asp:Timer ID="Timer1" runat="server" Interval="120000" 
  OnTick="Timer1_Tick">
</asp:Timer>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
  <Triggers>
    <asp:AsyncPostBackTrigger ControlID="Timer1" 
        EventName="Tick" />
    </Triggers>
    <ContentTemplate>
      <asp:Label ID="Label1" runat="server" ></asp:Label>
  </ContentTemplate>
</asp:UpdatePanel>