Tutorial · JavaScript

Unix timestamps in JavaScript

Date.now() vs new Date() vs performance.now()

Date.now() mirrors the number inside a Date object for "now" and is stable for persistence as long as you stay below Number.MAX_SAFE_INTEGER. new Date(isoString) invokes the spec parsing algorithm — some ISO variants are implementation-dependent; always supply full offsets. performance.now() starts at navigation and is not coupled to wall time; ideal for animation frames, wrong for databases.

const ms = Date.now();
const d = new Date(ms);
console.log(d.toISOString());

const t0 = performance.now();
await fetch('/api/profile');
console.log(`fetch ms (monotonic): ${(performance.now() - t0).toFixed(1)}`);

Temporal API direction

Temporal separates absolute instants, time zones, and calendars. Polyfills such as @js-temporal/polyfill let you experiment before full engine support. Adoption checklist: bundle size, SSR parity, and serialization format to your backend (often still RFC 3339 strings).

// npm i @js-temporal/polyfill
import { Temporal } from '@js-temporal/polyfill';

const inst = Temporal.Now.instant();
console.log(inst.epochMilliseconds);

Intl.DateTimeFormat

const fmt = new Intl.DateTimeFormat('en-US', {
  timeZone: 'Asia/Tokyo',
  dateStyle: 'full',
  timeStyle: 'long',
});
console.log(fmt.format(new Date(1713794701123)));

Common mistakes

• Month index starts at 0 in Date.UTC and Date constructor.
• Parsing date-only strings may default to UTC vs local — specify behavior in lint rules.
• Mixing seconds and ms silently — enforce length or string prefixes in APIs.

Library comparison

Key takeaways

• Always label persisted numbers with units; JS cannot do that for you.
• Prefer ISO strings on the wire when debugging; compact to ints only after validation.
• Temporal + polyfill is the long-term escape hatch from Date quirks.
• Server render: never assume client clock matches build server.
• Compare our precision guide before expanding schema.