use a javascript in wordpress plugin

Yes. You can :)

If you have a look at wp_register_script() on the Codex, you can see the last param of the function is $in_footer, if you set this to true the script will load in the footer rather than the header. So that's how you'll get it to be in the footer.

You don't need to nest the py_enqueue_script function inside the add_time function. In actually, fact I think that might cause you problems because the wp_enqueue_scripts action won't get called until wp_footer. You'd be better off taking it out of add_time. The final script would look something like the following. This will only work IF this code is called before wp_footer()

Also, I've noticed the function names you have given to the add_action function are missing quotes. These must be strings (or an array when working with classes).

<?php
if(get_option('show_content')) { 

    function py_enqueue_script () {
        // Note, check the link in the above text to see what the extra params do
        wp_register_script( 'timescript', plugin_url('../time.js', __FILE__), null, null, true);
        wp_enqueue_script( 'timescript'); 
    } // end py_enqueue_script
    add_action('wp_enqueue_scripts','py_enqueue_script');

    function add_time() {
        echo '<div id="txt">' . '</div>';
        // add script tut pro word plugin dev ch12.3
    } // end show add_time
    add_action("wp_footer",'add_time');

} // end if