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
$preSaleDate = $_product->getAvailabilityDate();//product attribute
if($preSaleDate) {
$format = 'Y-m-d H:i:s'; //current format
$formatted_date = DateTime::createFromFormat($format, $preSaleDate)->format('m/d/Y');
$dateReal = Mage::app()->getLocale()->date($formatted_date
,Zend_Date::DATE_SHORT, null, false);
$format = 'long'; // short, long, medium, full
$dueDate = Mage::helper('core')->formatDate($dateReal, $format, false);
echo $dueDate;// will get same day as original $preSaleDate
}
This solved my problem, don't know whether it is the right way to do it or not but i was more concerned about solving it
if($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);
}