// check whether date/time is valid or has already passed
    $date_array=getdate(strtotime($form_valuess['next']['date']));
 
    if (checkdate( $date_array['mon'], $date_array['mday'], $date_array['year'] == TRUE )) {
        $next_send=mktime($form_values['next']['hour'],0,0,$date_array['mon'], $date_array['mday'], $date_array['year']);
        
        if ($next_send<= time()) {
		           form_set_error('next', t('The date/time has already passed ')); 
		} 
	  }
		else {
		 	form_set_error('next', t('The date entered is wrong, Please make sure to enter the date in following format: mm/dd/yy.'));
       
		 }
		 
    $form['sendtime'] = array('#type' => 'value', '#value' => $next_send);
    form_set_value($form['sendtime'], $next_send);