What is worth noting in the snippet below is the usage of the 'onchange'=> remote_function( ... you can use the remote_function in many places to call other functions ... like in the "update", "complete" etc...

<div id="js_updating">Stand by..</div>
<?php echo form_tag('/module/action', 'method=get class=simpleForm') ?>
Please select the day:
<?php
echo input_date_tag('day', 'now', array('rich' => true, 'readonly'=>true, 'onchange'=> remote_function( array(
      'update'   => 'Area To Update (DIV TAG)',
      'url'      => 'MODULE/ACTION',
      'loading'  => "Element.show('js_updating')",
      'complete' => "Element.hide('js_updating')"
  ))));
 
?>
 
</form>