@return a deadline after which this selectable can expect to receive
a notification (via the {@link #expired()} method that indicates
that the deadline has past. The deadline is expressed in the
same format as {@link System#currentTimeMillis()}. Returning
a deadline of zero (or a negative number) indicates that the
selectable does not wish to be notified of expiry.
@return a deadline after which this selectable can expect to receive a notification (via the {@link #expired()} method that indicates that the deadline has past. The deadline is expressed in the same format as {@link System#currentTimeMillis()}. Returning a deadline of zero (or a negative number) indicates that the selectable does not wish to be notified of expiry.