custom.php

<?php

add_filter('wp_nav_menu_items','add_custom_in_menu', 10, 2);
function add_custom_in_menu( $items, $args ) {
    if( $args->theme_location == 'primary')  {
		
        $items .=  '<-- Your content here -->';

	}
    return $items;
}

date.php

<?php

add_filter('wp_nav_menu_items','add_todaysdate_in_menu', 10, 2);
function add_todaysdate_in_menu( $items, $args ) {
    if( $args->theme_location == 'primary')  {
		
		$todaysdate = date('l jS F Y');
        $items .=  '<li>' . $todaysdate .  '</li>';

	}
    return $items;
}

login-out.php

<?php

add_filter( 'wp_nav_menu_items', 'add_loginout_link', 10, 2 );
function add_loginout_link( $items, $args ) {
    if (is_user_logged_in() && $args->theme_location == 'primary') {
        $items .= '<li><a href="'. wp_logout_url() .'">Log Out</a></li>';
    }
    elseif (!is_user_logged_in() && $args->theme_location == 'primary') {
        $items .= '<li><a href="'. site_url('wp-login.php') .'">Log In</a></li>';
    }
    return $items;
}

searchbox.php

<?php

add_filter('wp_nav_menu_items','add_search_box_to_menu', 10, 2);
function add_search_box_to_menu( $items, $args ) {
    if( $args->theme_location == 'primary' )
        return $items."<li class='menu-header-search'><form action='".site_url()."' id='searchform' method='get'><input type='text' name='s' id='s' placeholder='Search'></form></li>";

    return $items;
}