Using ajax tool kit then work as below code 
You got timer:
ASPX
<form
id="form2"
runat="server">
    
<asp:ScriptManager
ID="ScriptManager1"
runat="server"
/>
       
<asp:Timer
runat="server"
id="UpdateTimer"
interval="1000"
ontick="UpdateTimer_Tick"
/>
       
<asp:UpdatePanel
runat="server"
id="TimedPanel"
updatemode="Conditional">
           
<Triggers>
               
<asp:AsyncPostBackTrigger
controlid="UpdateTimer"
eventname="Tick"
/>
           
</Triggers>
           
<ContentTemplate>
               
<asp:Label
runat="server"
id="DateStampLabel"
/>
           
</ContentTemplate>
       
</asp:UpdatePanel>
    
</form>
Aspx.cs
 
using 
System;
using 
System.Collections.Generic;
using 
System.Linq;
using 
System.Web;
using 
System.Web.UI;
using 
System.Web.UI.WebControls;
 
namespace 
WebApplication1
{
    
public partial 
class _4 : System.Web.UI.Page
    {
       
protected void 
Page_Load(object sender,
EventArgs e)
        {
        }
       
protected void 
UpdateTimer_Tick(object sender,
EventArgs e)
        {
            
DateStampLabel.Text = DateTime.Now.ToString();
        }
    }
}