private boolean validateHeaderNotAheadOfCurrentSystemTime(final long timestamp) {
    final long timestampMargin =
        TimeUnit.SECONDS.convert(System.currentTimeMillis(), TimeUnit.MILLISECONDS)
            + acceptableClockDriftSeconds;
    if (Long.compareUnsigned(timestamp, timestampMargin) > 0) {
      LOG.info(
          "Invalid block header: timestamp {} is greater than the timestamp margin {}",
          timestamp,
          timestampMargin);
      return false;
    }
    return true;
  }
Responsible for ensuring the timestamp of a block is not more than "acceptableClockDriftSeconds"
into the future.
ethereum/core/src/main/java/org/hyperledger/besu/ethereum/mainnet/headervalidationrules/TimestampBoundedByFutureParameter.java