functions.php

<?php

// Don't include the above

add_filter( 'wp_nav_menu_items', 'prefix_change_nav_date_format', 15, 2 );
/**
 * Filter the Primary Navigation menu items in Genesis, to change the date format.
 *
 * @author Gary Jones
 * @link   http://www.studiopress.com/forums/topic/abbreviate-month-in-primary-navigation-bar-enterprise/
 *
 * @uses genesis_get_option() Get navigation extras settings.
 *
 * @param string   $menu HTML string of list items.
 * @param stdClass $args Menu arguments.
 *
 * @return string Amended HTML string of list items.
 */
function prefix_change_nav_date_format( $menu, stdClass $args ) {
	if ( ! genesis_get_option( 'nav_extras' ) || 'primary' !== $args->theme_location )
		return $menu;

	if ( 'date' !== genesis_get_option( 'nav_extras' ) )
		return $menu;

	$format = 'j M Y'; // Change this, as per http://php.net/manual/en/function.date.php

	return preg_replace(
		'#\<li class="right date"\>(.*)\</li\>#',
		'<li class="right date">' . date_i18n( $format ) . '</li>',
		$menu
	);
}