Add reading time to post content with a custom shortcode.

reading-time-by-custom-shortcode.php

<?php

// [reading_time]
// [reading_time post_id="..."]
add_shortcode( 'reading_time', function($atts){
	
  $a = shortcode_atts( array(
    'post_id' => false,
	), $atts );
	
  $postId = $a['post_id'] ? intval($a['post_id']) : get_the_ID()
		
  if(empty($postId)) return "";
	
  // https://gist.github.com/EdwardBock/fa659b67bcf4f568289d0c49c175f42b
  return reading_time_get_content($postId);
});