下面是一个简单的WordPress插件，它可以在主页底部显示今日码字数、网站所有文章总字数、网站运行时间和今天的年月日。插件使用WordPress内置的计数器和时间函数，因此它是轻量级的，不需要连接到外部服务或API。你只需要把这个插件的代码保存为一个PHP文件，然后将它上传到WordPress插件目录中即可。

<?php
/**
 * Plugin Name: Custom Stats
 * Plugin URI: http://example.com/
 * Description: 这个插件用于在网站主页底部显示统计信息。
 * Version: 1.0
 * Author: Your Name
 * Author URI: http://example.com/
 */

function custom_stats() {
    // Count the words written today
    $today = date('Y-m-d');
    $args = array(
        'post_type' => 'post',
        'post_status' => 'publish',
        'date_query' => array(
            array(
                'after' => $today,
            ),
        ),
    );
    $today_query = new WP_Query($args);
    $word_count_today = 0;
    while ($today_query->have_posts()) {
        $today_query->the_post();
        $word_count_today += str_word_count(get_the_content());
    }
    wp_reset_postdata();

    // Count the total words on the site
    $args = array(
        'post_type' => 'post',
        'post_status' => 'publish',
    );
    $all_query = new WP_Query($args);
    $word_count_all = 0;
    while ($all_query->have_posts()) {
        $all_query->the_post();
        $word_count_all += str_word_count(get_the_content());
    }
    wp_reset_postdata();

    // Calculate the site's uptime
    $site_start_date = get_option('site_start_date');
    $site_uptime = human_time_diff(strtotime($site_start_date), current_time('timestamp'));

    // Display the stats
    echo '<div style="text-align: center; font-size: 16px;">';
    echo '<p>今日字数：' . $word_count_today . '字</p>';
    echo '<p>总字数：' . $word_count_all . '字</p>';
    echo '<p>运行时间：' . $site_uptime . '</p>';
    echo '<p>今天是：' . date('Y年m月d日') . '</p>';
    echo '</div>';
}

function add_custom_stats() {
    add_action('wp_footer', 'custom_stats');
}

add_action('init', 'add_custom_stats');

function site_start_date_activation() {
    if (!get_option('site_start_date')) {
        add_option('site_start_date', current_time('mysql'));
    }
}

register_activation_hook(__FILE__, 'site_start_date_activation');