code: public static Days daysBetween(ReadableInstant start, ReadableInstant end) {
          int amount = BaseSingleFieldPeriod.between(start, end, DurationFieldType.days());
          return Days.days(amount);
      }
docs: Creates a <code>Days</code> representing the number of whole days
  between the two specified datetimes. This method correctly handles
  any daylight savings time changes that may occur during the interval.
  
  @param start  the start instant, must not be null
  @param end  the end instant, must not be null
  @return the period in days
  @throws IllegalArgumentException if the instants are null or invalid