@Override
    public void setProducerId(long producerId) {
        if (producerId() == producerId) {
            return;
        }
        buffer.putLong(PRODUCER_ID_OFFSET, producerId);
        long crc = computeChecksum();
        ByteUtils.writeUnsignedInt(buffer, CRC_OFFSET, crc);
    }
Gets the base timestamp of the batch which is used to calculate the record timestamps from the deltas.

@return The base timestamp
clients/src/main/java/org/apache/kafka/common/record/DefaultRecordBatch.java