code: public static Hours hoursBetween(ReadableInstant start, ReadableInstant end) {
          int amount = BaseSingleFieldPeriod.between(start, end, DurationFieldType.hours());
          return Hours.hours(amount);
      }
docs: Creates a <code>Hours</code> representing the number of whole hours
  between the two specified datetimes.
  
  @param start  the start instant, must not be null
  @param end  the end instant, must not be null
  @return the period in hours
  @throws IllegalArgumentException if the instants are null or invalid