Date in JavaScript: representing, reading and formatting a moment

Handling time is where people get caught out most often, in every language. JavaScript adds two quirks of its own: months numbered from zero, and a time zone conversion that fires without warning.

Once those two points are understood, the Date object becomes predictable, and formatting is delegated entirely to the browser internationalization interface.

Definition

A Date object represents a single moment, stored internally as a number of milliseconds elapsed since January 1st 1970 at midnight UTC. Every reading method is just a different way of presenting that same number.

const d = new Date("2026-08-20T10:00:00Z");

console.log(d.getFullYear());  // 2026
console.log(d.getUTCMonth());  // 7 and not 8: months start at 0
console.log(d.getUTCDate());   // 20
console.log(d.toISOString());  // "2026-08-20T10:00:00.000Z"

console.log(typeof d.getTime());  // "number"

The new operator is mandatory. Without it, Date() returns a string instead of an object, which breaks everything downstream with no error message.

The time zone trap

How the date is written at creation time changes how it is interpreted.

The first line is the surprising one: a bare date is read as UTC, then displayed in the local zone. In New York it moves back a day as soon as it is printed with getDate(). The safe rule is to store in UTC and convert only when displaying.

Formatting and calculating

Never assemble a human-readable date by hand. Intl.DateTimeFormat knows the languages, the day-month orders and the month names.

const d = new Date("2026-08-20T10:00:00Z");

console.log(d.toLocaleDateString("en-US", {
  timeZone: "UTC", weekday: "long", day: "numeric", month: "long", year: "numeric"
}));
// "Thursday, August 20, 2026"

const start = new Date("2026-01-01T00:00:00Z");
const end = new Date("2026-01-08T00:00:00Z");
console.log((end - start) / 86400000);  // 7 days

The subtraction works because both objects convert to a number of milliseconds, a case of Coercion (type conversion) that for once does you a favor. Dividing by 86,400,000 gives a number of days.

Frequently asked questions

new Date() with no argument returns the current moment, and Date.now() returns that same moment directly in milliseconds. The second form is preferable for measuring a duration, since it avoids creating an object when only one number is needed.

The simplest route is setDate(d.getDate() + 7), which handles month boundaries and leap years on its own. Careful though, this method modifies the existing object rather than creating a new one: copy the date first if the original must stay intact.

Less and less. Intl covers formatting, which was the main need, and the Temporal API is progressively landing in browsers to handle zones and durations cleanly. A library still earns its place for parsing varied formats or advanced calendar arithmetic, much less so for displaying a date.