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