kalunder.php

<?php
/*
Plugin Name: Kalunder
Plugin URI: http://blog.josemcastaneda.comn
Description: Adds the shortcode kalunder so that you can display the WordPress calendar to any post, page or custom post type.
Author: Jose Castaneda
Version: 1.0.0
Author URI: https://blog.josemcastaneda.com
*/
add_shortcode( 'kalunder', 'jc_get_cal' );
function jc_get_cal( $atts ) {
	// set a unique ID for the calendar.
	$id = uniqid( 'wp-calendar-' );

	//get the calendar.
	$cal = get_calendar( false, false );

	// replace the ID.
	$cal = str_replace( 'id="wp-calendar"', 'id="' . $id . '"', $cal );

	// Set a wrapper.
	$out = sprintf( '<div class="calendar_wrap" id="calendar-%1$s">%2$s</div>',
		get_the_ID(),
		$cal
	);

	// finally return it.
	return $out;
}