session timeout problem

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ASP.NET

I am using the following code in Session_Start() function:

Session_Start() 
{   
  HttpContext context = HttpContext.Current;
  HttpCookieCollection cookies = context.Request.Cookies;
  if (cookies["starttime"] == null)
{
HttpCookie cookie = new HttpCookie("starttime",DateTime.Now.ToString());
cookie.Path = "/";
context.Response.Cookies.Add(cookie);
}
else
{
context.Response.Redirect("~/ErrorPages/SessionExpired.aspx", true);
}
} 

Due to this code users are facing session time out problem. This problem is not consistent so it's difficult to track. Also i am new to ASP.NET. So please provide me the solution for this problem.

Also I have set the session timeout 60 minutes in Web.config file and i am using the windows authentication mode.