<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def parse_duration(duration, timestamp=None):
 """ Interprets a ISO8601 duration value relative to a given timestamp. :param duration: The duration, as a string. :type: string :param timestamp: The unix timestamp we should apply the duration to. Optional, default to the current time. :type: string :return: The new timestamp, after the duration is applied. :rtype: int """

        assert isinstance(duration, basestring)
        assert timestamp is None or isinstance(timestamp, int)

        timedelta = duration_parser(duration)
        if timestamp is None:
            data = datetime.utcnow() + timedelta
        else:
            data = datetime.utcfromtimestamp(timestamp) + timedelta
        return calendar.timegm(data.utctimetuple())