code: @Override
      public int compareTo(UtcInstant otherInstant) {
          int cmp = Long.compare(mjDay, otherInstant.mjDay);
          if (cmp != 0) {
              return cmp;
          }
          return Long.compare(nanoOfDay, otherInstant.nanoOfDay);
      }
docs: Compares this instant to another based on the time-line.
  <p>
  The comparison is based first on the Modified Julian Day, then on the nano-of-day.
  It is "consistent with equals", as defined by {@link Comparable}.
  
  @param otherInstant  the other instant to compare to, not null
  @return the comparator value, negative if less, positive if greater