Problem in filtering data grid with Start Date and End date selection in DD-MMM-YYYY format

Hi,

I'm able to filter my data grid with Start date and End date selection but only with DD/MM/YYYY format . However I must need to have DD/MMM/YYYY format for my grid . When I'm applying my filter on this format getting error "Cannot access a property or method of a null object reference". I don't know what's wrong . Can someone point me what's wrong with DD/MMM/YYYY format or how to handle it in Flex.

My filter code ,

private function dateRangeFilter():void {

arrColl.filterFunction =doDateFilter
arrColl.refresh();
}

public function doDateFilter(item:Object):Boolean
{

var dt : Date = DateField.stringToDate(item.dob,"DD/MMM/YYYY");
var d1:Date=dateStart.selectedDate;
var d2:Date=dateEnd.selectedDate;
var d3:Number=dt.getTime();
var date1Timestamp:Number = d1.getTime();
var date2Timestamp:Number = d2.getTime();

if ( d3 >= date1Timestamp && d3 <= date2Timestamp)
{
return true;
}
return false;
}

My arraycollection data ,

name="User A" dob="11/Jan/2010"
name="User B" dob="18/Jan/2010"
name="User C" dob="25/Jan/2010"
name="User E" dob="01/Feb/2010"
name="User F" dob="21/Feb/2010"
name="User G" dob="01/Mar/2010"
name="User H" dob="21/Mar/2010"
name="User I" dob="17/Mar/2010"
name="User J" dob="29/Apr/2010"
name="User K" dob="16/Apr/2010"
name="User L" dob="08/May/2010"
name="User M" dob="27/May/2010"

I appreciate someone help on this .