DATE FIELD INSERTION THROUGH AR NOT WORKING

[eluser]Unknown[/eluser]
I have written following code to get the date value from a form having date text box having id and name 'datepicker'. But it does not insert the InvDate properly in MySQL. It just inserts 0000-00-00 in my mysql backend. Kindly suggest based upon my following code.

Codes:-
$this->load->database();
$data= array(
'InvId' => $_POST['InvId'] ,
'InvTyp' => 'S' ,
'InvDate' => $_POST['datepicker']
);
$this->db->insert('tmpinvoice', $data);