Código para el Widget de Entradas Recientes

Echa un vistazo a wp-includes\default-widgets.php.

/**
 * Clase del Widget Recent_Posts
 *
 * @since 2.8.0
 */
class WP_Widget_Recent_Posts extends WP_Widget {

    function __construct() {
        $widget_ops = array('classname' => 'widget_recent_entries', 'description' => __( "Las publicaciones más recientes de tu sitio") );
        parent::__construct('recent-posts', __('Publicaciones Recientes'), $widget_ops);
        $this->alt_option_name = 'widget_recent_entries';

        add_action( 'save_post', array($this, 'flush_widget_cache') );
        add_action( 'deleted_post', array($this, 'flush_widget_cache') );
        add_action( 'switch_theme', array($this, 'flush_widget_cache') );
    }

    function widget($args, $instance) {
        $cache = wp_cache_get('widget_recent_posts', 'widget');

        if ( !is_array($cache) )
            $cache = array();

        if ( ! isset( $args['widget_id'] ) )
            $args['widget_id'] = $this->id;

        if ( isset( $cache[ $args['widget_id'] ] ) ) {
            echo $cache[ $args['widget_id'] ];
            return;
        }

        ob_start();
        extract($args);

        $title = ( ! empty( $instance['title'] ) ) ? $instance['title'] : __( 'Publicaciones Recientes' );
        $title = apply_filters( 'widget_title', $title, $instance, $this->id_base );
        $number = ( ! empty( $instance['number'] ) ) ? absint( $instance['number'] ) : 10;
        if ( ! $number )
            $number = 10;
        $show_date = isset( $instance['show_date'] ) ? $instance['show_date'] : false;

        $r = new WP_Query( apply_filters( 'widget_posts_args', array( 'posts_per_page' => $number, 'no_found_rows' => true, 'post_status' => 'publish', 'ignore_sticky_posts' => true ) ) );
        if ($r->have_posts()) :
?>
        <?php echo $before_widget; ?>
        <?php if ( $title ) echo $before_title . $title . $after_title; ?>
        <ul>
        <?php while ( $r->have_posts() ) : $r->the_post(); ?>
            <li>
                <a href="<?php the_permalink() ?>" title="<?php echo esc_attr( get_the_title() ? get_the_title() : get_the_ID() ); ?>"><?php if ( get_the_title() ) the_title(); else the_ID(); ?></a>
            <?php if ( $show_date ) : ?>
                <span class="post-date"><?php echo get_the_date(); ?></span>
            <?php endif; ?>
            </li>
        <?php endwhile; ?>
        </ul>
        <?php echo $after_widget; ?>
<?php
        // Reinicia el global $the_post ya que esta consulta lo habrá sobrescrito
        wp_reset_postdata();

        endif;

        $cache[$args['widget_id']] = ob_get_flush();
        wp_cache_set('widget_recent_posts', $cache, 'widget');
    }

    function update( $new_instance, $old_instance ) {
        $instance = $old_instance;
        $instance['title'] = strip_tags($new_instance['title']);
        $instance['number'] = (int) $new_instance['number'];
        $instance['show_date'] = (bool) $new_instance['show_date'];
        $this->flush_widget_cache();

        $alloptions = wp_cache_get( 'alloptions', 'options' );
        if ( isset($alloptions['widget_recent_entries']) )
            delete_option('widget_recent_entries');

        return $instance;
    }

    function flush_widget_cache() {
        wp_cache_delete('widget_recent_posts', 'widget');
    }

    function form( $instance ) {
        $title     = isset( $instance['title'] ) ? esc_attr( $instance['title'] ) : '';
        $number    = isset( $instance['number'] ) ? absint( $instance['number'] ) : 5;
        $show_date = isset( $instance['show_date'] ) ? (bool) $instance['show_date'] : false;
?>
        <p><label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Título:' ); ?></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 $title; ?>" /></p>

        <p><label for="<?php echo $this->get_field_id( 'number' ); ?>"><?php _e( 'Número de publicaciones a mostrar:' ); ?></label>
        <input id="<?php echo $this->get_field_id( 'number' ); ?>" name="<?php echo $this->get_field_name( 'number' ); ?>" type="text" value="<?php echo $number; ?>" size="3" /></p>

        <p><input class="checkbox" type="checkbox" <?php checked( $show_date ); ?> id="<?php echo $this->get_field_id( 'show_date' ); ?>" name="<?php echo $this->get_field_name( 'show_date' ); ?>" />
        <label for="<?php echo $this->get_field_id( 'show_date' ); ?>"><?php _e( '¿Mostrar fecha de publicación?' ); ?></label></p>
<?php
    }
}

Si modificas este Widget, asegúrate de hacer un fork y registrar tu Widget modificado como un nuevo Widget.

Editar

Cómo hacer un fork de un Widget del núcleo

Comienza con la entrada del Codex sobre la API de Widgets.

1. Renombra el Widget

   Cambia esto:

   class WP_Widget_Recent_Posts extends WP_Widget {}
   

   ...por esto:

   class wpse97411_Widget_Recent_Posts extends WP_Widget {}
   

2. Modifica la clase del Widget según tus necesidades

3. Registra tu Widget:

   function wpse97413_register_custom_widgets() {
       register_widget( 'wpse97411_Widget_Recent_Posts' );
   }
   add_action( 'widgets_init', 'wpse97413_register_custom_widgets' );