set selected date in ajax calendar extender to textbox value (on focus)

using asp.net 4.0, ajaxtoolkit latest version.  have calendarextender control id="dtcalendar" TargetControlID="txtDt" which is textbox.
in general, if i select any date in calendarextendar (say 01/01/2011), it populates in txtDt. but if i change txtDt value to 01/31/2011 and moved to another control and came back to txtDt, the calendarextender shows the value that i selected before (i.e. 01/01/2011 not 01/31/2011). so i added the OnClientShowing event for calendarextender control like OnClientShowing="setEnteredDate"
function setEnteredDate(sender,args)
{
var txt=document.getElementById(sender._textbox._element.id);
if (txt!=null)
 sender.set_selectedDate=txt.value
 
}
Getting javascript error and couldnt select date anymore in calendarextender. so basically want to select date in calendar and want to populate in textbox and if i enter date in textbox, next time if i come back it s ...