please help me how can I use date _validate()

Hai

I have following code

function date_check_form()
{
$form['date_chk'] = array(
'#type' => 'date',
'#required' => $field->required
);
$form['submit']=array('#type'=>'submit','#value'=>'Select Date');
return $form;
}

please help me how can I use date _validate() to validate date I have selected ..

Regards

Kumar