WordPress – Sort Posts on Modified Date

How to sort posts on modified date (instead of creation date)?

Edit the functions.php file (in your theme)

function twentyeleven_posted_on() {
    printf( __( 'Modified on   by  ', 'twentyeleven' ),
        esc_url( get_permalink() ),
        esc_attr( get_the_modified_time() ),
        esc_attr( get_the_modified_date( 'c' ) ),
        esc_html( get_the_modified_date() ),
        esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
        sprintf( esc_attr__( 'View all posts by %s', 'twentyeleven' ), get_the_author() ),
        esc_html( get_the_author() )
    );
}

For the twentyfourteen theme (create a functions.php file):

if ( ! function_exists( 'twentyfourteen_posted_on' ) ) :
/**
 * Print HTML with meta information for the current post-date/time and author.
 *
 * @since Twenty Fourteen 1.0
 */
function twentyfourteen_posted_on() {
    if ( is_sticky() && is_home() && ! is_paged() ) {
        echo '<span class="featured-post">' . __( 'Sticky', 'twentyfourteen' ) . '</span>';
    }

    // Set up and print post meta information.
    printf( '<span class="entry-date"><a href="%1$s" rel="bookmark"><time class="entry-date" datetime="%2$s">%3$s</time></a></span>',
        esc_url( get_permalink() ),
        esc_attr( get_the_modified_date( 'c' ) ),
        esc_html( get_the_modified_date() )
    );
}
endif;

Edit index.php (in your theme):

if ( have_posts() ) :
    // Start the Loop.
    # RvG
    global $query_string;
    query_posts($query_string . '&orderby=modified&order=desc');    
    while ( have_posts() ) : the_post();

Resources:
http://wordpress.org/support/topic/sorting-posts-by-modified-date-not-creation-date