code: public static String formatPeriod(final long startMillis, final long endMillis, final String format) {
          return formatPeriod(startMillis, endMillis, format, true, TimeZone.getDefault());
      }
docs: <p>Formats the time gap as a string, using the specified format.
  Padding the left hand side of numbers with zeroes is optional.
  
  @param startMillis  the start of the duration
  @param endMillis  the end of the duration
  @param format  the way in which to format the duration, not null
  @return the formatted duration, not null
  @throws java.lang.IllegalArgumentException if startMillis is greater than endMillis