setTimeout Calling Callback Too Early

* **Version**: v8.15.1 * **Platform**: Darwin Charlies-MacBook-Pro.local 18.2.0 Darwin Kernel Version 18.2.0: Thu Dec 20 20:46:53 PST 2018; root:xnu-4903.241.1~1/RELEASE_X86_64 x86_64 --- I have the following code in Node.js. ```js const timeout = (ms) => new Promise((resolve) => setTimeout(resolve, ms)); ``` I'm trying to test this code with the following test: ```js it("Should wait for given time before resolving", async () => { const MS = 100; const start = process.hrtime(); await timeout(MS); const diff = process.hrtime(start); expect(((diff[0] * NS_PER_SEC) + diff[1]) / 1000000).to.at.least(MS); }); ``` The problem is sometimes (rarely), this test fails: Should wait for given time before resolving: AssertionError: expected 99.595337 to be at least 100 + expected - actual -99.595337 +100 Obviously this is some type of timing issue with Node.js or something. If anything I expect `await timeout(MS);` to take slightly longer than `MS`. In no case do I expect it to take less time. What is it about the internals of JavaScript/Node.js that causes this to happen? This occurred on macOS 10.14.3 running Node.js version 8.15.1.