docs: private Timestamp plus(long secondsToAdd, long nanosToAdd) {
      if ((secondsToAdd | nanosToAdd) == 0) {
        return this;
      }
      long epochSec = TimeUtils.checkedAdd(getSeconds(), secondsToAdd);
      epochSec = TimeUtils.checkedAdd(epochSec, nanosToAdd / NANOS_PER_SECOND);
      nanosToAdd = nanosToAdd % NANOS_PER_SECOND;
      long nanoAdjustment = getNanos() + nanosToAdd; // safe int + NANOS_PER_SECOND
      return ofEpochSecond(epochSec, nanoAdjustment);
    }