code: public static String formatPeriodISO(final long startMillis, final long endMillis) {
          return formatPeriod(startMillis, endMillis, ISO_EXTENDED_FORMAT_PATTERN, false, TimeZone.getDefault());
      }
docs: <p>Formats the time gap as a string.</p>
  
  <p>The format used is the ISO 8601 period format.</p>
  
  @param startMillis  the start of the duration to format
  @param endMillis  the end of the duration to format
  @return the formatted duration, not null
  @throws java.lang.IllegalArgumentException if startMillis is greater than endMillis