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+import { Observable } from '../Observable';
+import { SchedulerLike } from '../types';
+import { async as asyncScheduler } from '../scheduler/async';
+import { isScheduler } from '../util/isScheduler';
+import { isValidDate } from '../util/isDate';
+
+/**
+ * Creates an observable that will wait for a specified time period, or exact date, before
+ * emitting the number 0.
+ *
+ * <span class="informal">Used to emit a notification after a delay.</span>
+ *
+ * This observable is useful for creating delays in code, or racing against other values
+ * for ad-hoc timeouts.
+ *
+ * The `delay` is specified by default in milliseconds, however providing a custom scheduler could
+ * create a different behavior.
+ *
+ * ## Examples
+ *
+ * Wait 3 seconds and start another observable
+ *
+ * You might want to use `timer` to delay subscription to an
+ * observable by a set amount of time. Here we use a timer with
+ * {@link concatMapTo} or {@link concatMap} in order to wait
+ * a few seconds and start a subscription to a source.
+ *
+ * ```ts
+ * import { of, timer, concatMap } from 'rxjs';
+ *
+ * // This could be any observable
+ * const source = of(1, 2, 3);
+ *
+ * timer(3000)
+ *   .pipe(concatMap(() => source))
+ *   .subscribe(console.log);
+ * ```
+ *
+ * Take all values until the start of the next minute
+ *
+ * Using a `Date` as the trigger for the first emission, you can
+ * do things like wait until midnight to fire an event, or in this case,
+ * wait until a new minute starts (chosen so the example wouldn't take
+ * too long to run) in order to stop watching a stream. Leveraging
+ * {@link takeUntil}.
+ *
+ * ```ts
+ * import { interval, takeUntil, timer } from 'rxjs';
+ *
+ * // Build a Date object that marks the
+ * // next minute.
+ * const currentDate = new Date();
+ * const startOfNextMinute = new Date(
+ *   currentDate.getFullYear(),
+ *   currentDate.getMonth(),
+ *   currentDate.getDate(),
+ *   currentDate.getHours(),
+ *   currentDate.getMinutes() + 1
+ * );
+ *
+ * // This could be any observable stream
+ * const source = interval(1000);
+ *
+ * const result = source.pipe(
+ *   takeUntil(timer(startOfNextMinute))
+ * );
+ *
+ * result.subscribe(console.log);
+ * ```
+ *
+ * ### Known Limitations
+ *
+ * - The {@link asyncScheduler} uses `setTimeout` which has limitations for how far in the future it can be scheduled.
+ *
+ * - If a `scheduler` is provided that returns a timestamp other than an epoch from `now()`, and
+ * a `Date` object is passed to the `dueTime` argument, the calculation for when the first emission
+ * should occur will be incorrect. In this case, it would be best to do your own calculations
+ * ahead of time, and pass a `number` in as the `dueTime`.
+ *
+ * @param due If a `number`, the amount of time in milliseconds to wait before emitting.
+ * If a `Date`, the exact time at which to emit.
+ * @param scheduler The scheduler to use to schedule the delay. Defaults to {@link asyncScheduler}.
+ */
+export function timer(due: number | Date, scheduler?: SchedulerLike): Observable<0>;
+
+/**
+ * Creates an observable that starts an interval after a specified delay, emitting incrementing numbers -- starting at `0` --
+ * on each interval after words.
+ *
+ * The `delay` and `intervalDuration` are specified by default in milliseconds, however providing a custom scheduler could
+ * create a different behavior.
+ *
+ * ## Example
+ *
+ * ### Start an interval that starts right away
+ *
+ * Since {@link interval} waits for the passed delay before starting,
+ * sometimes that's not ideal. You may want to start an interval immediately.
+ * `timer` works well for this. Here we have both side-by-side so you can
+ * see them in comparison.
+ *
+ * Note that this observable will never complete.
+ *
+ * ```ts
+ * import { timer, interval } from 'rxjs';
+ *
+ * timer(0, 1000).subscribe(n => console.log('timer', n));
+ * interval(1000).subscribe(n => console.log('interval', n));
+ * ```
+ *
+ * ### Known Limitations
+ *
+ * - The {@link asyncScheduler} uses `setTimeout` which has limitations for how far in the future it can be scheduled.
+ *
+ * - If a `scheduler` is provided that returns a timestamp other than an epoch from `now()`, and
+ * a `Date` object is passed to the `dueTime` argument, the calculation for when the first emission
+ * should occur will be incorrect. In this case, it would be best to do your own calculations
+ * ahead of time, and pass a `number` in as the `startDue`.
+ * @param startDue If a `number`, is the time to wait before starting the interval.
+ * If a `Date`, is the exact time at which to start the interval.
+ * @param intervalDuration The delay between each value emitted in the interval. Passing a
+ * negative number here will result in immediate completion after the first value is emitted, as though
+ * no `intervalDuration` was passed at all.
+ * @param scheduler The scheduler to use to schedule the delay. Defaults to {@link asyncScheduler}.
+ */
+export function timer(startDue: number | Date, intervalDuration: number, scheduler?: SchedulerLike): Observable<number>;
+
+/**
+ * @deprecated The signature allowing `undefined` to be passed for `intervalDuration` will be removed in v8. Use the `timer(dueTime, scheduler?)` signature instead.
+ */
+export function timer(dueTime: number | Date, unused: undefined, scheduler?: SchedulerLike): Observable<0>;
+
+export function timer(
+  dueTime: number | Date = 0,
+  intervalOrScheduler?: number | SchedulerLike,
+  scheduler: SchedulerLike = asyncScheduler
+): Observable<number> {
+  // Since negative intervalDuration is treated as though no
+  // interval was specified at all, we start with a negative number.
+  let intervalDuration = -1;
+
+  if (intervalOrScheduler != null) {
+    // If we have a second argument, and it's a scheduler,
+    // override the scheduler we had defaulted. Otherwise,
+    // it must be an interval.
+    if (isScheduler(intervalOrScheduler)) {
+      scheduler = intervalOrScheduler;
+    } else {
+      // Note that this *could* be negative, in which case
+      // it's like not passing an intervalDuration at all.
+      intervalDuration = intervalOrScheduler;
+    }
+  }
+
+  return new Observable((subscriber) => {
+    // If a valid date is passed, calculate how long to wait before
+    // executing the first value... otherwise, if it's a number just schedule
+    // that many milliseconds (or scheduler-specified unit size) in the future.
+    let due = isValidDate(dueTime) ? +dueTime - scheduler!.now() : dueTime;
+
+    if (due < 0) {
+      // Ensure we don't schedule in the future.
+      due = 0;
+    }
+
+    // The incrementing value we emit.
+    let n = 0;
+
+    // Start the timer.
+    return scheduler.schedule(function () {
+      if (!subscriber.closed) {
+        // Emit the next value and increment.
+        subscriber.next(n++);
+
+        if (0 <= intervalDuration) {
+          // If we have a interval after the initial timer,
+          // reschedule with the period.
+          this.schedule(undefined, intervalDuration);
+        } else {
+          // We didn't have an interval. So just complete.
+          subscriber.complete();
+        }
+      }
+    }, due);
+  });
+}