public function isFree(DateTimeInterface $start, DatetimeInterface $end)
    {
        foreach ($this->select('FREEBUSY') as $freebusy) {
            // We are only interested in FBTYPE=BUSY (the default),
            // FBTYPE=BUSY-TENTATIVE or FBTYPE=BUSY-UNAVAILABLE.
            if (isset($freebusy['FBTYPE']) && 'BUSY' !== strtoupper(substr((string) $freebusy['FBTYPE'], 0, 4))) {
                continue;
            }

            // The freebusy component can hold more than 1 value, separated by
            // commas.
            $periods = explode(',', (string) $freebusy);

            foreach ($periods as $period) {
                // Every period is formatted as [start]/[end]. The start is an
                // absolute UTC time, the end may be an absolute UTC time, or
                // duration (relative) value.
                list($busyStart, $busyEnd) = explode('/', $period);

                $busyStart = VObject\DateTimeParser::parse($busyStart);
                $busyEnd = VObject\DateTimeParser::parse($busyEnd);
                if ($busyEnd instanceof \DateInterval) {
                    $busyEnd = $busyStart->add($busyEnd);
                }

                if ($start < $busyEnd && $end > $busyStart) {
                    return false;
                }
            }
        }

        return true;
    }