<%@ WebHandler Language="C#" Class="SessionHeartbeatHttpHandler" %>

using System;
using System.Web;
using System.Web.SessionState;

public class SessionHeartbeatHttpHandler : IHttpHandler, IRequiresSessionState
{
    public bool IsReusable { get { return false; } }

    public void ProcessRequest(HttpContext context)
    {
        context.Session["Heartbeat"] = DateTime.Now;
    }
}