Setting Date Value from my Date Picker

See more:

ASP.NET

Javascript

I'm trying to get a date picker on my asp.net webpage. I've been able to run the same code using a master page
and it works, but now I'm trying to get it to work without a master pager and I'm getting error message:
Error: Unable to get value of the property 'aspnetForm.ctl00_txtDOB': object is null or undefined



On my Default.aspx page....

<script language="javascript" type="text/javascript">
    function calendarPicker(strField) {
        window.open('date_picker.aspx?field=' + strField, 'calendarPopup', 'width=250,height=190,resizable=yes');
    }


        <label>DOB:</label><asp:TextBox ID="txtDOB" runat="server"></asp:TextBox>
       <img src="images/calendar.gif" alt="Pick To Date" onclick="calendarPicker('aspnetForm.ctl00_txtDOB');"/>

I already tried onclick="calendarPicker('txtDOB');"/>


My date_picker.aspx.cs

private void Calendar1_DayRender(object sender, System.Web.UI.WebControls.DayRenderEventArgs e)
       {

           // Clear the link from this day
           e.Cell.Controls.Clear();

           // Add the custom link
           System.Web.UI.HtmlControls.HtmlGenericControl Link = new System.Web.UI.HtmlControls.HtmlGenericControl();
           Link.TagName = "a";
           Link.InnerText = e.Day.DayNumberText;
           Link.Attributes.Add("href", String.Format("JavaScript:window.opener.document.{0}.value = \'{1:d}\'; window.close();", Request.QueryString["field"], e.Day.Date));




           // By default, this will highlight today's date.
           if (e.Day.IsSelected)
           {
               Link.Attributes.Add("style", this.Calendar1.SelectedDayStyle.ToString());
           }



           // Now add our custom link to the page
           e.Cell.Controls.Add(Link);

       }