function et_last_modified_date_blog( $the_date ) {
    if ( 'post' === get_post_type() ) {
        $the_time = get_post_time( 'His' );
        $the_modified = get_post_modified_time( 'His' );

        $last_modified =  sprintf( __( 'Last updated %s', 'Divi' ), esc_html( get_post_modified_time( 'M j, Y' ) ) );
        $date = $the_modified !== $the_time ? $last_modified : get_post_time( 'M j, Y' );

        return $date;
    }
}
add_action( 'get_the_date', 'et_last_modified_date_blog' );
add_action( 'get_the_time', 'et_last_modified_date_blog' );

$last_modified =  sprintf( __( '<strong>Last updated %s</strong>', 'Divi' ), esc_html( get_post_modified_time( 'M j, Y' ) ) );