diff --git a/packages/rxjs/spec/operators/buffer-spec.ts b/packages/rxjs/spec/operators/buffer-spec.ts
index debb7fa9d5..81bfefb16f 100644
--- a/packages/rxjs/spec/operators/buffer-spec.ts
+++ b/packages/rxjs/spec/operators/buffer-spec.ts
@@ -3,6 +3,7 @@ import { EMPTY, NEVER, throwError, of, Subject, interval } from 'rxjs';
 import { TestScheduler } from 'rxjs/testing';
 import { observableMatcher } from '../helpers/observableMatcher';
 import { expect } from 'chai';
+import * as sinon from 'sinon';
 
 /** @test {buffer} */
 describe('Observable.prototype.buffer', () => {
@@ -325,6 +326,8 @@ describe('Observable.prototype.buffer', () => {
   });
 
   it('should buffer when Promise resolves', (done) => {
+    const sandbox = sinon.createSandbox();
+    const fakeTimers = sandbox.useFakeTimers();
     const e1 = interval(3).pipe(take(5));
     const expected = [
       [0, 1],
@@ -338,9 +341,12 @@ describe('Observable.prototype.buffer', () => {
       error: () => done(new Error('should not be called')),
       complete: () => {
         expect(expected.length).to.equal(0);
+        sandbox.restore();
         done();
       },
     });
+
+    fakeTimers.tickAsync(15);
   });
 
   it('should raise error when Promise rejects', (done) => {
diff --git a/packages/rxjs/spec/operators/skipUntil-spec.ts b/packages/rxjs/spec/operators/skipUntil-spec.ts
index 65d5695ecd..e8c28eda8f 100644
--- a/packages/rxjs/spec/operators/skipUntil-spec.ts
+++ b/packages/rxjs/spec/operators/skipUntil-spec.ts
@@ -4,6 +4,7 @@ import { skipUntil, mergeMap, take } from 'rxjs/operators';
 import { asInteropObservable } from '../helpers/interop-helper';
 import { TestScheduler } from 'rxjs/testing';
 import { observableMatcher } from '../helpers/observableMatcher';
+import * as sinon from 'sinon';
 
 /** @test {skipUntil} */
 describe('skipUntil', () => {
@@ -369,6 +370,8 @@ describe('skipUntil', () => {
   });
 
   it('should skip until Promise resolves', (done) => {
+    const sandbox = sinon.createSandbox();
+    const fakeTimers = sandbox.useFakeTimers();
     const e1 = interval(3).pipe(take(5));
     const expected = [2, 3, 4];
 
@@ -379,9 +382,12 @@ describe('skipUntil', () => {
       error: () => done(new Error('should not be called')),
       complete: () => {
         expect(expected.length).to.equal(0);
+        sandbox.restore();
         done();
       },
     });
+
+    fakeTimers.tickAsync(15);
   });
 
   it('should raise error when Promise rejects', (done) => {
diff --git a/packages/rxjs/spec/operators/window-spec.ts b/packages/rxjs/spec/operators/window-spec.ts
index fb67cc7bac..ae58843649 100644
--- a/packages/rxjs/spec/operators/window-spec.ts
+++ b/packages/rxjs/spec/operators/window-spec.ts
@@ -4,6 +4,7 @@ import type { Observable} from 'rxjs';
 import { EMPTY, of, interval } from 'rxjs';
 import { observableMatcher } from '../helpers/observableMatcher';
 import { expect } from 'chai';
+import * as sinon from 'sinon';
 
 /** @test {window} */
 describe('window', () => {
@@ -284,6 +285,8 @@ describe('window', () => {
   });
 
   it('should window when Promise resolves', (done) => {
+    const sandbox = sinon.createSandbox();
+    const fakeTimers = sandbox.useFakeTimers();
     const e1 = interval(3).pipe(take(5));
     let pos = 0;
     const result: number[][] = [[], []];
@@ -302,9 +305,11 @@ describe('window', () => {
       error: () => done(new Error('should not be called')),
       complete: () => {
         expect(result).to.deep.equal(expected);
+        sandbox.restore();
         done();
       },
     });
+    fakeTimers.tickAsync(15);
   });
 
   it('should raise error when Promise rejects', (done) => {