BLOCK_NO_CACHE seems to have no effect in certain situations

Hi,

I have just spent a day trying to fix a caching-related issue with a block on one of our client websites. It's a weird one which I can't explain, but I probably don't understand fully how Drupal caching works, so perhaps this is a documentation issue rather than a code one. I've marked this up as base system, rather than block.module, because the issue seems to be caching in general rather than blocks, but mark it back to block.module if I'm wrong.

Anyway, I used the jQuery Countdown module (http://drupal.org/project/jquery_countdown) to create a block loading a CCK date field on a node and showing a counter down to the display date. Page cache is set to Normal and Block cache is enabled. The code looks like this:

// FIRST THE HOOK_BLOCK
// Note, BLOCK_NO_CACHE is explicitly set:
function economistconferences_features_block($op = 'list', $delta = 0) {
  switch ($op) {
    case 'list':
      $blocks['countdown']['info'] = t('Event countdown');
      $blocks['countdown']['cache'] = BLOCK_NO_CACHE;
      return $blocks;
    break;
    
    case 'view':
      switch ($delta) {
        case 'countdown':
          $block['subject'] = t('Countdown');
          $block['content'] = theme('economistconferences_features_countdown');
          return $block;
      }
    break;
  }
}

// THEN THE THEME FUNCTION CALLED
// This just loads our date, does some simple maths and then calls a theme function
// over at the jQuery Countdown module for return:
function theme_economistconferences_features_countdown() {
  if (module_exists('jquery_countdown') && arg(0) == 'node' && is_numeric(arg(1))) {
    $node = node_load(arg(1));
    $field = $node->field_events_date;
    if ($field[0]['value']) {      
      $difference = strtotime($field[0]['value']) - time();
      
      if ($difference < 0) {
        $passed = 1;
        $difference = abs($difference);
      }
      
      if (!$passed) {
        return theme('jquery_countdown', array(
          'until' => $difference,
          'format' => 'DHMS',
          'description' => t('to go until event'),
          'layout' => '<div class="countdown-clock"><div class="day">{dn}</div><div class="clock">{hnn} {sep} {mnn} {sep} {snn}</div></div><div class="clearfix"></div><div class="countdown-text"><div class="text">{dl}</div><div class="desc">{desc}</div></div><div class="clearfix"></div>',
        ));
      }      
    }
  }
  else {
    return t('To use this block you must install and enable !link', array('!link' => l(t('jQuery Countdown'), 'http://drupal.org/project/jquery_countdown')));
  } 
}

This code works fine for logged in users, without exception. That makes sense, I guess, because the page cache is not doing anything for logged in users. And even if the block is cached, it shouldn't matter because the date is normally static data anyway.

However, for anonymous users, after a period of time (I guess the cache expiry time) the block stops rendering correctly. It doesn't even add the JS for jQuery Countdown any more (which happens in the theme function called within my theme function). It just renders a grey box. Again, this kinda makes sense - with page caching on, perhaps the additional function would not be fired, because Drupal would use the stale JavaScript, except that block is not containing stale data - it's complete empty. Now that makes NO sense to me...?

And it gets weirder. If you disable block cache (which, until today, I thought had no effect on anonymous users when page cache was enabled) the problem goes away. I cannot begin to tell you why, it just does. All of a sudden, that block starts rendering perfectly. Which is mad for a couple of reasons:

1. Block cache shouldn't matter because page cache is on and the block should be cached in the page?
2. Anyway, the block is set to BLOCK_NO_CACHE, so why is it being cached at all - it should already be disabled for that block!

So, muchos weirdiness. Any ideas?