code: public static Weeks weeksBetween(ReadableInstant start, ReadableInstant end) {
          int amount = BaseSingleFieldPeriod.between(start, end, DurationFieldType.weeks());
          return Weeks.weeks(amount);
      }
docs: Creates a <code>Weeks</code> representing the number of whole weeks
  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 weeks
  @throws IllegalArgumentException if the instants are null or invalid