code: private static Duration parseDuration(String periodString) {
      DateTime zeroEpoc = new DateTime(0);
      return new Duration(zeroEpoc, zeroEpoc.plus(ISOPeriodFormat.standard().parsePeriod(periodString)));
    }
docs: Since months and years can have arbitrary days, joda time does not allow conversion of a period string containing
  months or years to a duration. Hence we calculate the duration using 1970 01:01:00:00:00 UTC as a reference time.
  
  <p>
  <code>
  (1970 01:01:00:00:00 + P2Y) - 1970 01:01:00:00:00 = Duration for 2 years
  </code>
  </p>
  
  @param periodString
  @return duration for this period.