I am trying to use AJAX with Drupal 6. How do I determine the URL I am trying to contact?
The URL you use is the one that delivers you the data you want.  If you want to grab the server time you can create a short PHP script that gets the info and then make that info available to JS using Drupal.settings.

For example:

// Create the array that is required by drupal_add_js()
$server_time = array(
  'serverTime' =&gt; date("h:i:sa"),
);

// This is what makes the variable available to JS
drupal_add_js(array('time' =&gt; $server_time), 'setting');


Then, in your.js:

var time = Drupal.settings.time.serverTime;