form_set_error to "inner" form element

Hello!

I've written a little validation for an event-contenttype that displays an error, if the date is in the past. Here we go:

function _mymodule_event_node_form_validate($form, &$form_state) {
  $creationTS = time();
  $eventTS = $form_state['values']['field_event_datetime']['0']['value'] + $form_state['values']['field_event_datetime']['0']['offset'];
  if(($eventTS - $creationTS) < 0) {
     form_set_error('field_event_datetime', t('Don't use a date in the past.'));
  }
}

So this works, but I am looking for a way to set the error not for the hole field_event_datetime form element, but for a part of it (just the date-field). Printing out $form shows this Array:

Array
(
    [#id] => node-form
    [nid] => Array
        (
            [#type] => value
            [#value] => 
            [#post] => Array
                (
                    [title] => mein termin :D
                    [field_termin_datetime] => Array
                        (
                            [value] => Array
                                (
                                    [date] => 02.03.2010
                                    [time] => 20:00
                                )

So I thought I can use form_set_error('field_event_datetime[value][date]', t('Don't use a date in the past.')); or something similar - but had no luck. Can you tell me how to adress this date form element?! The HTML-Code of it is also with [] brackets: <input type="text" maxlength="30" name="field_termin_datetime[value][date]" id="edit-field-termin-datetime-value-datepicker-popup-0" size="20" value="02.03.2010" class="form-text" />

Thanks for any hint - regards, Freddy