This functionality breaks the panels module.
Create a webform with a date field and this popup enabled. Add it to a panels page (full not teaser). Save.
Return to the editing of the panel page and pop open firebug. The following error is returned:
$calendar is undefined
var startYear = $calendar.class...bform-calendar-start-(\d+).*/, '$1');
Now when you edit the settings of any panels component (the modal that pops up) and hit the finish button, the modal will never close and firebug will display the above noted error.