help! how to save value of date type form to table filed ?

I have a form names 'expiry' that users can choose a dealine date for their post as below

 $form['expiry']=array(
   '#title' => t('Deadline'),
   '#type' => 'date_popup',
   '#description' => t('Set the deadline.'),
   '#default_value' =>$node->expiry,
   );

My problem is how to save the date in to the table field when implementing the hook_ insert

I tried below but failed,the date users selected just can't be saved into the table field 'expiry'.

mymodule_insert ($node) {

 db_query("INSERT INTO {exampletable} (nid,expiry) VALUES ( %d,%d) ", $node->nid,$node->expiry);

}

I don't know why,please help