code: public static Weeks weeksBetween(ReadablePartial start, ReadablePartial end) {
          if (start instanceof LocalDate && end instanceof LocalDate)   {
              Chronology chrono = DateTimeUtils.getChronology(start.getChronology());
              int weeks = chrono.weeks().getDifference(
                      ((LocalDate) end).getLocalMillis(), ((LocalDate) start).getLocalMillis());
              return Weeks.weeks(weeks);
          }
          int amount = BaseSingleFieldPeriod.between(start, end, ZERO);
          return Weeks.weeks(amount);
      }
docs: Creates a <code>Weeks</code> representing the number of whole weeks
  between the two specified partial datetimes.
  <p>
  The two partials must contain the same fields, for example you can specify
  two <code>LocalDate</code> objects.
  
  @param start  the start partial date, must not be null
  @param end  the end partial date, must not be null
  @return the period in weeks
  @throws IllegalArgumentException if the partials are null or invalid