i followed steps on this link to add a date field to my custom module :
http://magentomechanic.blogspot.com/2010/01/to-add-custom-date-field-in-custom.html
Every
This solved my problem, don't know whether it is the right way to do it or not but i was more concerned about solving it
if($data['start_date'] != NULL )
{
$start_time_array = explode("/", $data['start_date']);
$start_time = $start_time_array[2]."-".$start_time_array[0]."-".$start_time_array[1]." 00:00:00";
$model->setStartDate($start_time);
}