src/main/java/io/reactivex/Maybe.java
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public static Maybe<Long> timer(long delay, TimeUnit unit, Scheduler scheduler) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");

        return RxJavaPlugins.onAssembly(new MaybeTimer(Math.max(0L, delay), unit, scheduler));
    }
@CheckReturnValue
    @NonNull
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public static Maybe<Long> timer(long delay, TimeUnit unit, Scheduler scheduler) {
        ObjectHelper.requireNonNull(unit, "unit is null");
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");

        return RxJavaPlugins.onAssembly(new MaybeTimer(Math.max(0L, delay), unit, scheduler));
    }
Returns a Maybe that emits {@code 0L} after a specified delay on a specified Scheduler.
<p>
<img width="640" height="200" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/timer.s.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>You specify which {@link Scheduler} this operator will use.</dd>
</dl>

@param delay
the initial delay before emitting a single 0L
@param unit
time units to use for {@code delay}
@param scheduler
the {@link Scheduler} to use for scheduling the item
@return a Maybe that emits {@code 0L} after a specified delay, on a specified Scheduler
@see <a href="http://reactivex.io/documentation/operators/timer.html">ReactiveX operators documentation: Timer</a>