All in One Event Calendar - Adding filters + functions to Twig themes For more reading on uses of functions vs. filters in Twig: http://twig.sensiolabs.org/doc/advanced.html

functions.php

function add_twig_functions_and_filters() {
  global $ai1ec_front_controller;
  $loader = $ai1ec_front_controller->return_registry(true)->get('theme.loader');
  $twig = $loader->get_twig_instance(false, true);
  function foo($date) {
      if(mktime() > strtotime($date->__toString)) {
        return true;
      }
      else return false;
  }
  $expired = new Twig_SimpleFunction('expired', 'foo');
  $twig->addFunction($expired);
  
  function bar($string) {
    return $string.'ringding';
  }
  $filter = new Twig_SimpleFilter('mFilter', 'bar');
  $twig->addFilter($filter);
}
add_action('init', 'add_twig_functions_and_filters');

theme.twig

{% if expired(event.get('start')) %}
  <span>{{'Expired'|mFilter}}</span> //Will output 'Expiredringding'
{% endif %}

Thank you very much, this is just what I was looking for! I was nearly giving up, after looking at the plugin code.