Calendar Date select pop up error

Hi there,
I have a small form within a form that I want to click a link that pops up a calendar and when a date is selected in the calendar the date will populate a textbox in the form.  

Here's my form code:
<form name="frmCalendar">
 <input name="txtDate" type="text" />
<a href="javascript:calendar_window=window.open('calendar.aspx?formname=frmCalendar.txtDate','calendar_window');calendar_window.focus()">Calendar</a>
</form>

Here's my calendar code:
Private Sub Calendar1_SelectionChanged(sender As Object, e As System.EventArgs)
Dim strjscript as string = "<script language=""javascript"">"
strjscript = strjscript & "window.opener." & HttpContext.Current.Request.QueryString("formname") & ".value = '" & Calendar1.SelectedDate & "';window.close();"
strjscript = strjscript & "</script" & ">" 'Don't Ask, Tool Bug
Literal1.text = strjscript
End Sub

I'm getting the following error when I click on a date in the calendar:

Error: 'window.open.frmCalendar.txtDate' is null or not an object.  

It seems to be in this line:
strjscript = strjscript & "window.opener." & HttpContext.Current.Request.QueryString("formname") & ".value = '" & Calendar1.SelectedDate & "';window.close();"

Any help is apprecaited.

Thanks,
Full
FullWilliamsAsked:
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SammyCommented:
Take a look at this solution http://www.experts-exchange.com/Programming/Programming_Languages/Dot_Net/ASP_DOT_NET/Q_21912671.html
it doesnt use a popup instead it uses a hidden div. clean and easy to use

HTH
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