JavaScript timestamps

Seconds and milliseconds

The Date constructor expects milliseconds when passed a number. Multiply Unix seconds by 1,000.

const seconds = 1711972800;
const date = new Date(seconds * 1000);
console.log(date.toISOString());
// 2024-04-01T12:00:00.000Z

const nowMs = Date.now();
const nowSeconds = Math.floor(nowMs / 1000);

Parse a date with an explicit UTC offset

const date = new Date('2024-04-01T14:00:00+02:00');
if (Number.isNaN(date.getTime())) {
  throw new Error('Invalid date');
}
console.log(date.getTime()); // 1711972800000

A date and time without an offset may be interpreted in the browser time zone. Use Z for UTC or an offset such as +02:00. Validating Date is not enough to reject every impossible calendar date: some are normalized.

Display a geographical time zone

const formatter = new Intl.DateTimeFormat('fr-FR', {
  dateStyle: 'full',
  timeStyle: 'long',
  timeZone: 'Europe/Paris'
});
console.log(formatter.format(date));

Precision and large values

Date represents instants to the millisecond. Nanosecond timestamps commonly exceed the integer precision of Number: read them as strings, then use BigInt. Converting to Date does not preserve fractions of a millisecond.

References

MDN — Date · BigInt

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English version published September 26, 2026.