src/main/java/io/reactivex/Flowable.java
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Flowable<T> delay(long delay, TimeUnit unit, Scheduler scheduler) {
        return delay(delay, unit, scheduler, false);
    }
@CheckReturnValue
    @BackpressureSupport(BackpressureKind.FULL)
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Flowable<T> delay(long delay, TimeUnit unit, Scheduler scheduler) {
        return delay(delay, unit, scheduler, false);
    }
Returns a Flowable that emits the items emitted by the source Publisher shifted forward in time by a
specified delay. Error notifications from the source Publisher are not delayed.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/delay.s.png" alt="">
<dl>
<dt><b>Backpressure:</b></dt>
<dd>The operator doesn't interfere with the backpressure behavior which is determined by the source {@code Publisher}.</dd>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param delay
the delay to shift the source by
@param unit
the time unit of {@code delay}
@param scheduler
the {@link Scheduler} to use for delaying
@return the source Publisher shifted in time by the specified delay
@see <a href="http://reactivex.io/documentation/operators/delay.html">ReactiveX operators documentation: Delay</a>