I had that error too.
This is how I fixed it:
http://www.silverstripe.org/all-other-modules/show/282832#post282835
open \event_calendar\code\CalendarUI.class.php
go to line 117
replace:
public function setAnchorEnd($date)
protected
with:
public function setAnchorEnd($date)
{
$this->anchor_end = new sfDate(CalendarUtil::getDateToString($date));
}