Add reading time to post content with a custom Gutenberg block.

reading-time-by-gutenberg-block.js

import {registerBlockType} from '@wordpress/blocks';
import ServerSideRender from '@wordpress/server-side-render';

registerBlockType( 'public-function-org/reading-time', {
    title: "Reading time",
    icon: 'clock',
    category: 'common',
    edit: () => {
        return <ServerSideRender
          block="public-function-org/reading-times"
        />
    },
} );

reading-time-by-gutenberg-block.php

<?php

add_action('init', function(){
  register_block_type(
    'public-function-org/reading-time',
    [
      "render_callback" => function( $attributes, $editorContent){
        
        $postId = get_the_ID();
        if(empty($postId)) return "";
        
        // https://gist.github.com/EdwardBock/fa659b67bcf4f568289d0c49c175f42b
        return reading_time_get_content($postId);
      }
    ]
  )
});