code: public static long plus(long packedDateTime, long amountToAdd, TemporalUnit unit) {
  
          LocalDateTime dateTime = asLocalDateTime(packedDateTime);
          return pack(dateTime.plus(amountToAdd, unit));
      }
docs: Returns the given packedDateTime with amtToAdd of temporal units added
  
  TODO(lwhite): Replace with a native implementation that doesn't convert everything to LocalDateTime