code: void writeExternal(DataOutput out) throws IOException {
          Ser.writeEpochSec(toEpochSecond(), out);
          Ser.writeOffset(offsetBefore, out);
          Ser.writeOffset(offsetAfter, out);
      }
docs: Writes the state to the stream.
  
  @param out  the output stream, not null
  @throws IOException if an error occurs