Custom form field to create a select list with year numbers

yearlist.php

<?php
/**
 * @version    1.0.0
 * @author     Rene Kreijveld <r.kreijveld@dsd.nl>
 * @copyright  2019 DSD Business Internet
 * @license    GNU General Public License version 2 or later; see LICENSE.txt
 *
 * Example form xml definition:
 * <field name="established"
 *     type="yearlist"
 *     start="1950"
 *     end="9999"
 *     defaultvalue="before 1950"
 *     defaulttext="Before 1950"
 *     label="Year of establishment"
 *     description="Choose the year this company was established"
 * />
 */

defined('JPATH_BASE') or die;

use \Joomla\CMS\HTML\HTMLHelper;

/**
 * Supports an HTML select list of years
 *
 * @since  1.6
 */
class JFormFieldYearList extends \Joomla\CMS\Form\FormField
{
	/**
	 * The form field type.
	 *
	 * @var        string
	 * @since    1.6
	 */
	protected $type = 'yearlist';

	/**
	 * Method to get the field input markup.
	 *
	 * @return    string    The field input markup.
	 *
	 * @since    1.6
	 */
	protected function getInput()
	{
		$this->required = (string) $this->getAttribute('required', 0);

		// the year where the list starts with
		$this->start = (int) $this->getAttribute('start', 1950);

		// the year where the list ends with
		// if end = 9999, the current year will be used
		$this->end = (string) $this->getAttribute('end', 9999);

		// default value to be used at the start of the list
		// degfault text to be used at the start if the list
		$this->defaultvalue = (string) $this->getAttribute('defaultvalue');
		$this->defaulttext  = (string) $this->getAttribute('defaulttext');

		$input_options = 'class="' . $this->getAttribute('class') . '"';
		if ($this->required === "true")
		{
			$input_options .= 'required="required"';
		}

		// array that holds the options for the select list
		$options = array();

		// do we need to add a default value at the start of the list?
		if ($this->defaultvalue)
		{
			$options[] = HTMLHelper::_('select.option', $this->defaultvalue, $this->defaulttext);
		}

		if ($this->end === 9999)
		{
			$this->end = (int) date('Y');
		}

		// build list
		for ($i = $this->start; $i <= $this->end; $i++)
		{
			$options[] = HTMLHelper::_('select.option', (string) $i, (string) $i);
		}

		// create HTML for the select list
		$html = HTMLHelper::_('select.genericlist', $options, $this->name, $input_options, 'value', 'text', $this->value, $this->id);
		return $html;
	}
}