<?php
// Display value in frontoffice
    function display_value_time($content) {
        global $post;
        $value_time = get_post_meta($post->ID, '_my_field' , true);
        if($value_time != ''){
            if ($value_time < 2){
                $text = __( 'You will take less than a minute to read the article ', 'reading_time_domain' );
            }
            else{
                $text = __( 'Approximate length of article reading time : ', 'reading_time_domain' ). "<strong>" . $value_time." minutes </strong>";
            }
        return  "<div class='reading-time'>". $text. "</div>" . $content;
        }
        return $content;
    }
    add_filter('the_content', 'display_value_time');