public function diff(Twig_Environment $env, $date, $now = null)
    {
        // Convert both dates to DateTime instances.
        $date = twig_date_converter($env, $date);
        $now = twig_date_converter($env, $now);

        // Get the difference between the two DateTime objects.
        $diff = $date->diff($now);

        // Check for each interval if it appears in the $diff object.
        foreach (self::$units as $attribute => $unit) {
            $count = $diff->$attribute;

            if (0 !== $count) {
                return $this->getPluralizedInterval($count, $diff->invert, $unit);
            }
        }

        return '';
    }