Return dates this service period is active as a list of "YYYYMMDD".

def ActiveDates(self):
    """Return dates this service period is active as a list of "YYYYMMDD"."""
    (earliest, latest) = self.GetDateRange()
    if earliest is None:
      return []
    dates = []
    date_it = util.DateStringToDateObject(earliest)
    date_end = util.DateStringToDateObject(latest)
    delta = datetime.timedelta(days=1)
    while date_it <= date_end:
      date_it_string = date_it.strftime("%Y%m%d")
      if self.IsActiveOn(date_it_string, date_it):
        dates.append(date_it_string)
      date_it = date_it + delta
    return dates