How to Add Reading Time to Genesis Child Theme Without a Plugin

functions.php

/**
 * Estimated Reading Time
 *
 * Code Modified from Original Code Author:
 * https://medium.com/@nadeem4uwebtech/how-to-add-reading-time-in-wordpress-without-using-plugin-d2e8af7b1239
*/
function c8d_reading_time() {
    global $post;  // Declare global $post to access it within the function
    $content = get_post_field( 'post_content', $post->ID );
    $word_count = str_word_count( strip_tags( $content ) );
    $readingtime = ceil($word_count / 200);

    if ($readingtime == 1) {
        $timer = " minute";
    } else {
        $timer = " minutes";
    }
    $totalreadingtime = $readingtime . $timer;

    return $totalreadingtime;
}
add_shortcode('reading_time', 'c8d_reading_time');