code: public static Months monthsIn(ReadableInterval interval) {
          if (interval == null)   {
              return Months.ZERO;
          }
          int amount = BaseSingleFieldPeriod.between(interval.getStart(), interval.getEnd(), DurationFieldType.months());
          return Months.months(amount);
      }
docs: Creates a <code>Months</code> representing the number of whole months
  in the specified interval. This method correctly handles any daylight
  savings time changes that may occur during the interval.
  
  @param interval  the interval to extract months from, null returns zero
  @return the period in months
  @throws IllegalArgumentException if the partials are null or invalid