code: public static Weeks weeksIn(ReadableInterval interval) {
          if (interval == null)   {
              return Weeks.ZERO;
          }
          int amount = BaseSingleFieldPeriod.between(interval.getStart(), interval.getEnd(), DurationFieldType.weeks());
          return Weeks.weeks(amount);
      }
docs: Creates a <code>Weeks</code> representing the number of whole weeks
  in the specified interval.
  
  @param interval  the interval to extract weeks from, null returns zero
  @return the period in weeks
  @throws IllegalArgumentException if the partials are null or invalid