Functions to add to the functions.php to change "Top Posts" widget from Jetpack

modify-topposts01.php

<?php
/* MODIFICAMOS EL TIEMPO A CONSIDERAR PARA EL "TOP POSTS" */
function jetpackcustom_new_timeframe() {
    /* CONSIDERAREMOS 30 DÍAS */
    return '30';
}
add_filter( 'jetpack_top_posts_days', 'jetpackcustom_new_timeframe' );

/**
 * WIDGET TOP POSTS: AGREGAMOS LA FECHA AL FINAL
 */
function jetpackcustom_new_adddate( $post_id ) {
        $post_date = get_post_time(
                get_option( 'date_format' ),
                true,
                $post_id,
                true
        );
        printf( '<div class="top_posts_date widgets-list-layout-links">%s</div>', $post_date );
}
add_action( 'jetpack_widget_top_posts_after_post', 'jetpackcustom_new_adddate' );
?>