sorry for my mistake
chang
popUp = window.open('Calendar.aspx?formname=' + document.forms[0].name
to
popUp = window.open('PopUpVivek.aspx?formname=' + document.forms[0].name
and in PopUpVivek.aspx in source put
<asp:Calendar id="CalVivek" nSelectionChanged="Change_Date" runat="server" style="Z-INDEX: 101; LEFT: 8px; POSITION: absolute; TOP: 4px; font-size: 9pt; color: dimgray; font-family: arial;" onselectionchanged="CalVivek_SelectionChanged" Height="1px" Width="1px">
and in c# put
protected void CalVivek_SelectionChanged(object sender, System.EventArgs e)
{
if (Request.QueryString["textbox"] != "")
{
string strScript = "<script>window.opener.document.forms(0)." + Request.QueryString["textbox"].ToString() + ".value = '";
strScript += CalVivek.SelectedDate.ToString("yyyy/MM/dd");
strScript += "';self.close()";
strScript += "</" + "script>";
RegisterClientScriptBlock("Calendar_ChangeDate", strScript);
}
that is all
wait for your reply