using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;

public partial class Expt : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {


    }
    protected void Bttnadd_Click(object sender, EventArgs e)
    {
       FileUpload1.SaveAs(MapPath("~/img//"+DateTime.Now.ToString()+FileUpload1.FileName));

    }
}

Exception Details: System.Web.HttpException: ~/img//1/1/2011 1:47:52 PMWinter.jpg is not a valid virtual path.