If the start and end times of the job are defined, return a timedelta,
        else return None

def duration(self):
        """
        If the start and end times of the job are defined, return a timedelta,
        else return None
        """
        try:
            start, end = self.hmget('start', 'end')
            return parse(end) - parse(start)
        except:
            return None