Drupal snippet to validate whether a room (defined by a taxonomy term) has already been booked during the dates selected on a node form.

custom.module

<?php

/**
 * Implements hook_form_alter().
 *
 * Adds validation to Booking node forms.
 */
function custom_form_alter(&$form, &$form_state, $form_id) {
  if ($form_id == 'booking_node_form') {
    $form['#validate'][] = 'custom_booking_validate';
  }
}

/**
 * Validation function for Booking node forms.
 * Sends form_state values to custom_check_booking_availability().
 */
function custom_booking_validate($form, $form_state) {
  $data = array(
    'room' => $form_state['values']['field_room']['und'][0]['tid'],
    'start' => $form_state['values']['field_booking_date']['und'][0]['value'],
    'end' => $form_state['values']['field_booking_date']['und'][0]['value2'],
  );

  if (!isset($form_state['values']['field_booking_date']['und'][0]['timezone'])) {
    $data['start'] = gmdate('Y-m-d H:i:s', strtotime($data['start']));
    $data['end'] = gmdate('Y-m-d H:i:s', strtotime($data['end']));
  }

  if ($hit = custom_check_booking_availability($data)) {
    form_set_error('field_room', 'The following event(s) are already booked in that time slot:' . $hit);
    form_set_error('field_booking_date');
  }
}

/**
 * Validates availability of a Booking room, given a range of dates.
 * Returns any Bookings already reserved in the given room at the given times.
 */
function custom_check_booking_availability($data) {
  $query = db_select('node', 'n');

  $query->leftJoin('field_data_field_room', 'r', 'r.entity_id = n.nid');
  $query->leftJoin('field_data_field_booking_date', 'd', 'd.entity_id = n.nid');
  $query
    ->condition('n.type', 'booking')
    ->condition('n.status', 1)
    ->condition('r.field_room_tid', $data['room'])
    ->condition('d.field_booking_date_value', $data['end'], '<=')
    ->condition('d.field_booking_date_value2', $data['start'], '>=')
    ->fields('n', array('title', 'nid'));

  $result = $query->execute();

  $hit = '';
  foreach ($result as $row) {
    $hit .= '<br />' . l($row->title, 'node/' . $row->nid);
  }

  if ($hit != '') {
    return $hit;
  }
  else {
    return false;
  }
}