code: public long toEpochSecond(ZoneOffset offset) {
          Jdk8Methods.requireNonNull(offset, "offset");
          long epochDay = toLocalDate().toEpochDay();
          long secs = epochDay * 86400 + toLocalTime().toSecondOfDay();
          secs -= offset.getTotalSeconds();
          return secs;
      }
docs: Converts this date-time to the number of seconds from the epoch
  of 1970-01-01T00:00:00Z.
  <p>
  This combines this local date-time and the specified offset to calculate the
  epoch-second value, which is the number of elapsed seconds from 1970-01-01T00:00:00Z.
  Instants on the time-line after the epoch are positive, earlier are negative.
  
  @param offset  the offset to use for the conversion, not null
  @return the number of seconds from the epoch of 1970-01-01T00:00:00Z