JavaScript Dates and Times: The Date Object, Formatting and Time Zones

Dates appear in almost every application: order times, due dates, birthdays, "posted 3 days ago". JavaScript's built-in Date object handles them, with a few quirks worth knowing.

Creating dates#

const fromIso = new Date("2026-11-14T09:30:00Z");
const fromParts = new Date(Date.UTC(2026, 10, 14, 9, 30));   // month 10 = November!
const fromTimestamp = new Date(0);

console.log(fromIso.toISOString());
console.log(fromParts.toISOString());
console.log(fromTimestamp.toISOString());

2026-11-14T09:30:00.000Z
2026-11-14T09:30:00.000Z
1970-01-01T00:00:00.000Z

• An ISO 8601 string (YYYY-MM-DDTHH:mm:ssZ) is the safest way to create a date from text.
• Months are numbered from 0: January is 0 and December is 11. Days of the month start at 1. This trips up everyone.
• A timestamp is the number of milliseconds since 1 January 1970, 00:00 UTC.

new Date() with no arguments gives the current date and time, and Date.now() gives the current timestamp:

console.log(new Date());     // the current date and time
console.log(Date.now());     // milliseconds since 1970, e.g. for timing code

Reading parts of a date#

Each part has a local-time getter and a UTC getter:

const d = new Date("2026-11-14T09:30:00Z");

console.log(d.getUTCFullYear());
console.log(d.getUTCMonth());       // 0-based
console.log(d.getUTCDate());        // day of the month
console.log(d.getUTCDay());         // day of the week: 0 = Sunday
console.log(d.getUTCHours(), d.getUTCMinutes());
console.log(d.getTime());

2026
10
14
6
9 30
1794648600000

The local versions (getFullYear, getMonth, getHours and so on) use the time zone of the computer running the code.

Date arithmetic#

Dates are stored as timestamps, so subtracting two dates gives milliseconds:

const start = new Date("2026-10-24T00:00:00Z");
const exam = new Date("2026-12-01T00:00:00Z");

const msPerDay = 24 * 60 * 60 * 1000;
const days = Math.round((exam - start) / msPerDay);
console.log(`${days} days until the exam`);

const dueDate = new Date(start);
dueDate.setUTCDate(dueDate.getUTCDate() + 30);   // add 30 days
console.log(dueDate.toISOString().slice(0, 10));

38 days until the exam
2026-11-23

Setters such as setUTCDate roll over correctly: adding 30 days to 24 October lands in November. Note that setters change the Date object, which is why the example copies start first with new Date(start).

Comparing dates#

const a = new Date("2026-01-01T00:00:00Z");
const b = new Date("2026-01-01T00:00:00Z");

console.log(a < new Date("2026-06-01T00:00:00Z"));
console.log(a === b);
console.log(a.getTime() === b.getTime());

true
false
true

< and > work, but === compares object references, so compare getTime() values for equality.

Formatting dates for people#

toLocaleDateString and Intl.DateTimeFormat format dates for any language and region:

const d = new Date("2026-11-14T09:30:00Z");

console.log(d.toLocaleDateString("en-IN", { timeZone: "UTC" }));
console.log(d.toLocaleDateString("en-US", { timeZone: "UTC" }));
console.log(d.toLocaleDateString("en-GB", { dateStyle: "long", timeZone: "UTC" }));
console.log(
  new Intl.DateTimeFormat("en-IN", {
    weekday: "long",
    day: "numeric",
    month: "long",
    hour: "numeric",
    minute: "2-digit",
    timeZone: "Asia/Kolkata",
  }).format(d)
);

14/11/2026
11/14/2026
14 November 2026
Saturday, 14 November at 3:00 pm

The same moment is 09:30 in UTC and 15:00 in India (UTC+5:30), which is why the last line shows 3:00 pm. Always format dates with these APIs rather than building strings by hand. (The output above is from Node.js; small details such as commas and the word "at" come from the locale data built into each browser and can differ slightly between versions.)

Time zones#

A Date stores one moment in time. What clock time that moment shows depends on the time zone. A few rules avoid most bugs:

• Store and send dates in UTC, as ISO strings (toISOString()), especially in databases and APIs.
• Convert to local time only for display, with toLocaleString and a timeZone option when needed.
• Be careful with date-only strings: new Date("2026-11-14") is parsed as midnight UTC, which can show as 13 November in time zones behind UTC.

The Temporal API#

Because Date has so many quirks, JavaScript is gaining a new date and time API called Temporal, with separate types for plain dates, times, time zones and durations. It has started shipping in browsers but is not yet available everywhere. Until it is, Date with Intl (or a small library such as date-fns or Day.js) is the standard choice.

Practice#

const start = new Date("2026-09-01T00:00:00Z");
const end = new Date(start);
end.setUTCDate(end.getUTCDate() + 75);

const today = new Date("2026-10-24T00:00:00Z");
const msPerDay = 24 * 60 * 60 * 1000;

console.log(end.toISOString().slice(0, 10));
console.log(end.toLocaleDateString("en-IN", { weekday: "long", timeZone: "UTC" }));
console.log(Math.round((end - today) / msPerDay), "days left");

2026-11-15
Sunday
22 days left

Key takeaway#