code: public static ChronoUnit chronoUnit(TimeUnit unit) {
          Objects.requireNonNull(unit, "unit");
          switch (unit) {
              case NANOSECONDS:
                  return ChronoUnit.NANOS;
              case MICROSECONDS:
                  return ChronoUnit.MICROS;
              case MILLISECONDS:
                  return ChronoUnit.MILLIS;
              case SECONDS:
                  return ChronoUnit.SECONDS;
              case MINUTES:
                  return ChronoUnit.MINUTES;
              case HOURS:
                  return ChronoUnit.HOURS;
              case DAYS:
                  return ChronoUnit.DAYS;
              default:
                  throw new IllegalArgumentException("Unknown TimeUnit constant");
          }
      }
docs: Converts a {@code TimeUnit} to a {@code ChronoUnit}.
  <p>
  This handles the seven units declared in {@code TimeUnit}.
  
  @param unit  the unit to convert, not null
  @return the converted unit, not null