src/main/java/io/reactivex/Completable.java
@CheckReturnValue
    @Experimental
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable delaySubscription(long delay, TimeUnit unit, Scheduler scheduler) {
        return Completable.timer(delay, unit, scheduler).andThen(this);
    }
@CheckReturnValue
    @Experimental
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Completable delaySubscription(long delay, TimeUnit unit, Scheduler scheduler) {
        return Completable.timer(delay, unit, scheduler).andThen(this);
    }
Returns a Completable that delays the subscription to the source CompletableSource by a given amount of time,
both waiting and subscribing on a given Scheduler.
<p>
<img width="640" height="420" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Completable.delaySubscription.ts.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param delay     the time to delay the subscription
@param unit      the time unit of {@code delay}
@param scheduler the Scheduler on which the waiting and subscription will happen
@return a Completable that delays the subscription to the source CompletableSource by a given
amount, waiting and subscribing on the given Scheduler
@since 2.2.3 - experimental
@see <a href="http://reactivex.io/documentation/operators/delay.html">ReactiveX operators documentation: Delay</a>