Download > Install > Activate. To edit, use your favorite IDE or just edit within the WordPress plugin editor. Change the open and close times to suite your needs. Times are based on server time so adjust accordingly.

In the code below I am open 9-5 mountain or 16:00 - 11:59 UTC (server time). The last line of the code allows for the short code to be used within a WordPress menu which is displayed in my header.

This code can be pasted into the functions.php file of your child theme or, download and install the plugin for simple code activation.

/*
Plugin Name: Lightweight Plugins - Business Hours Indicator
Plugin URI: https://lightweightplugins.com/plugin/business-hours-indicator/
Description: A lightweight plugin that adds a little open / closed pill with colors.
Version: 1.6
Author: WP Maintenance Experts
Author URI: https://wpmaintenanceexperts.com/
License: GPLv2 or later
*/

function build_lighweight_business_hours_indicator_shortcode(){
$now = new Datetime("now");
$opentime = new DateTime('16:00');
$closetime = new DateTime('23:59');

if($now >= $opentime && $now <= $closetime){
    // between times
    $output = '
Open
';
} else {
    // not between times
    $output = '
Closed
';
}
// 	$output = '
' . $now . '
'; //uncomment line to see current server time
	return $output;
}
add_shortcode( 'lwopenclose', 'build_lighweight_business_hours_indicator_shortcode');

add_filter('wp_nav_menu_items', 'do_shortcode');