| 1 date.elements.inc | date_empty_end_date(&$element, &$form_state, &$input) | 
Helper function to clear out end date when not being used.
File
- core/modules/ date/ date.elements.inc, line 1823 
- Date forms and form themes and validation.
Code
function date_empty_end_date(&$element, &$form_state, &$input) {
  // If this is the end date and the option to show an end date has not been
  // selected, empty the end date to suppress validation errors and stop further
  // processing.
  $parents = $element['#parents'];
  $parent = array_pop($parents);
  if ($parent == 'value2') {
    $parent_values = backdrop_array_get_nested_value($form_state['values'], $parents);
    if (isset($parent_values['show_todate']) && $parent_values['show_todate'] != 1) {
      $input = array();
      form_set_value($element, NULL, $form_state);
    }
  }
}
