class DateElementBase

abstract class DateElementBase extends FormElementBase {
    
    /**
     * Specifies the start and end year to use as a date range.
     *
     * Handles a string like -3:+3 or 2001:2010 to describe a dynamic range of
     * minimum and maximum years to use in a date selector.
     *
     * Centers the range around the current year, if any, but expands it far enough
     * so it will pick up the year value in the field in case the value in the field
     * is outside the initial range.
     *
     * @param string $string
     *   A min and max year string like '-3:+1' or '2000:2010' or '2000:+3'.
     * @param object $date
     *   (optional) A date object to test as a default value. Defaults to NULL.
     *
     * @return array
     *   A numerically indexed array, containing the minimum and maximum year
     *   described by this pattern.
     */
    protected static function datetimeRangeYears($string, $date = NULL) {
        $datetime = new DrupalDateTime();
        $this_year = $datetime->format('Y');
        [
            $min_year,
            $max_year,
        ] = explode(':', $string);
        // Valid patterns would be -5:+5, 0:+1, 2008:2010.
        $plus_pattern = '@[\\+|\\-][0-9]{1,4}@';
        $year_pattern = '@^[0-9]{4}@';
        if (!preg_match($year_pattern, $min_year, $matches)) {
            if (preg_match($plus_pattern, $min_year, $matches)) {
                $min_year = $this_year + $matches[0];
            }
            else {
                $min_year = $this_year;
            }
        }
        if (!preg_match($year_pattern, $max_year, $matches)) {
            if (preg_match($plus_pattern, $max_year, $matches)) {
                $max_year = $this_year + $matches[0];
            }
            else {
                $max_year = $this_year;
            }
        }
        // We expect the $min year to be less than the $max year. Some custom values
        // for -99:+99 might not obey that.
        if ($min_year > $max_year) {
            $temp = $max_year;
            $max_year = $min_year;
            $min_year = $temp;
        }
        // If there is a current value, stretch the range to include it.
        $value_year = $date instanceof DrupalDateTime ? $date->format('Y') : '';
        if (!empty($value_year)) {
            $min_year = min($value_year, $min_year);
            $max_year = max($value_year, $max_year);
        }
        return [
            $min_year,
            $max_year,
        ];
    }
    
    /**
     * Returns the most relevant title of a datetime element.
     *
     * Since datetime form elements often consist of combined date and time fields
     * the element title might not be located on the element itself but on the
     * parent container element.
     *
     * @param array $element
     *   The element being processed.
     * @param array $complete_form
     *   The complete form structure.
     *
     * @return string
     *   The title.
     */
    protected static function getElementTitle($element, $complete_form) {
        $title = '';
        if (!empty($element['#title'])) {
            $title = $element['#title'];
        }
        else {
            $parents = $element['#array_parents'];
            array_pop($parents);
            $parent_element = NestedArray::getValue($complete_form, $parents);
            if (!empty($parent_element['#title'])) {
                $title = $parent_element['#title'];
            }
        }
        return $title;
    }

}