Retrieve unix timestamp from calendar

I made a patch for jsCalendar to make the Unix timestamp readily available when users choose a different date through the calendar. The timestamp is updated by the calendar itself and saved in a hidden field, and it can be read in the submit function by the $_POST variable (seems that the $form_values gets the original values for hidden fields, so the changes are available in $_POST only...). Here I explain my changes.
In the jscalendar.module I added the "timestamp" hidden field, and set the other fileds as disabled, as the submit doesn't need them.

function jscalendar_form_alter($form_id, &$form) {
  ........
      <strong>$settings = array('timestamp', 'ifFormat', 'showsTime', 'timeFormat');</strong>
      foreach ($settings as $setting) {
	if (isset($form[$key]['#jscalendar_' . $setting])) {
	  $form[$key.'_jscalendar']['#tree'] = true;
          $form[$key.'_jscalendar'][$setting] = array(
            '#type' => 'hidden',
            '#value' => $form[$key]['#jscalendar_' . $setting],
            <strong>'#disabled' => ($setting != 'timestamp'),  // disable values not necessary in form submit</strong>
          );
          unset($form[$key]['#jscalendar_' . $setting]);
        }
      }
  ........
}

In the jscalendar.js I passed the timestamp field as a parameter, and added the code to fire when the calendar is updated. The cal.date.print('%s') is the unix timestamp for the chosen date.

        Calendar.setup(
          {
            inputField  : this.id,
            ifFormat    : settings['ifFormat'],
            button      : id + '-button',
            showsTime   : settings['showsTime'],
            timeFormat  : settings['timeFormat'],
            timestamp    : this.form.elements[id + '_jscalendar-timestamp'],
            onUpdate    : calendarUpdated
          }
        );

function calendarUpdated(cal) {
  // update timestamp hidden value
  var timestamp = cal.params.timestamp;
  if (timestamp) timestamp.value = cal.date.print('%s');
}

This is an example with the applied changes ($t is the unix timestamp, used in the default value and in the #jscalendar_timestamp parameter):

$form['start_date'] = array(
'#title' => t('Start Date'),
'#type' => 'textfield',
'#default_value'=>format_date($t, 'custom', 'd/m/Y'),
'#attributes' => array('class' => 'jscalendar'),
'#jscalendar_ifFormat' => '%Y-%m-%d %H:%M',
'#jscalendar_showsTime' => 'false',
'#jscalendar_timestamp'=>$t,
(...)
);

In the form submit function the unix timestamp is available by the $_POST variable:

$start_date = $_POST['startdate_jscalendar']['timestamp'];