code: public long toEpochSecond() {
          long epochDay = toLocalDate().toEpochDay();
          long secs = epochDay * 86400 + toLocalTime().toSecondOfDay();
          secs -= getOffset().getTotalSeconds();
          return secs;
      }
docs: Converts this date-time to the number of seconds from the epoch
  of 1970-01-01T00:00:00Z.
  <p>
  This uses the {@linkplain #toLocalDateTime() local date-time} and
  {@linkplain #getOffset() 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.
  
  @return the number of seconds from the epoch of 1970-01-01T00:00:00Z