ASP.NET中网页的加载时间

protected void Application_BeginRequest(Object sender, EventArgs e)
{
Application["start"] = DateTime.Now ;
}

protected void Application_EndRequest(Object sender, EventArgs e)
{
System.TimeSpan aa = DateTime.Now -(System.DateTime) Application["start"];
Response.Write(aa.TotalSeconds.ToString("#00.000"));
}