Calling a Drupal 6 theme function

Hi,

I've been trying to use a function for theming a table with form information, and it doesn't even get called. Does anybody understand why the use of layouts/themes in drupal 6 it's not working the same than in version 5?

in my form function I'm calling: $form ['Activity']['time']['until']['#theme'] = 'theme_time2';

which is: function theme_time2 (&$form) {
//put the layouting for the selects until

$output .= "";
$output .= "Until:
";
$output .= "".theme('select', $form['dateday'])."";
$output .= "".theme('select', $form['datemonth'])."";
$output .= "".theme('select', $form['dateyear'])."";
$output .= "".theme('select', $form['hour'])."";
$output .= "".theme('select', $form['minute'])."";
$output .= "";
return $output;
}

If there's some documentation I can't find, I'd be grateful you could send it.

Marc