Jetpack's Top Posts Widget: add post date and post author below each post.

plugin.php

<?php
/**
 * Plugin Name: Top Posts Widget: add post date and post author below each post.
 * Plugin URI: http://jeremy.hu
 * Description: Top Posts Widget: add post date and post author below each post.
 * Version: 1.0
 * Author: Jeremy Herve
 * Author URI: http://jeremy.hu
 * Text Domain: jeherve
 * Domain Path: /languages/
 * License: GPL2
 */

function jeherve_add_date_author_top_posts( $post_id ) {

	$post_date = get_post_time(
		get_option( 'date_format' ),
		true,
		$post_id,
		true
	);

	$post_author      = get_post_field( 'post_author', $post_id );
	$post_author_name = get_the_author_meta( 'display_name', $post_author );

	printf(
		'On <span class="top_posts_date">%1$s</span>, by <span class="top_posts_author">%2$s</span>',
		$post_date,
		$post_author_name
	);
}
add_action( 'jetpack_widget_top_posts_after_post', 'jeherve_add_date_author_top_posts' );