WordPress/wp-includes/widgets/class-wp-widget-calendar.php

<?php
/**
 * Calendar widget class
 *
 * @since 2.8.0
 * @package WordPress
 * @subpackage Widgets
 */
class WP_Widget_Calendar extends WP_Widget {

	public function __construct() {
		$widget_ops = array('classname' => 'widget_calendar', 'description' => __( 'A calendar of your site&#8217;s Posts.') );
		parent::__construct('calendar', __('Calendar'), $widget_ops);
	}

	/**
	 * @param array $args
	 * @param array $instance
	 */
	public function widget( $args, $instance ) {
		/** This filter is documented in wp-includes/widgets/class-wp-widget-pages.php */
		$title = apply_filters( 'widget_title', empty( $instance['title'] ) ? '' : $instance['title'], $instance, $this->id_base );

		echo $args['before_widget'];
		if ( $title ) {
			echo $args['before_title'] . $title . $args['after_title'];
		}
		echo '<div id="calendar_wrap">';
		get_calendar();
		echo '</div>';
		echo $args['after_widget'];
	}

	/**
	 * @param array $new_instance
	 * @param array $old_instance
	 * @return array
	 */
	public function update( $new_instance, $old_instance ) {
		$instance = $old_instance;
		$instance['title'] = sanitize_text_field( $new_instance['title'] );

		return $instance;
	}

	/**
	 * @param array $instance
	 */
	public function form( $instance ) {
		$instance = wp_parse_args( (array) $instance, array( 'title' => '' ) );
		$title = sanitize_text_field( $instance['title'] );
?>
		<p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
		<input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo esc_attr($title); ?>" /></p>
<?php
	}
}