Date stores a single number — milliseconds since 1 Jan 1970 UTC — and has an awkward API. Intl formats dates, numbers and lists for any locale; Temporal is the modern replacement arriving now.

Creating dates

JavaScript

const epoch = new Date(0);
console.log(epoch.toISOString());
const utc = new Date(Date.UTC(2024, 0, 31, 12, 30));   // months are 0-based!
console.log(utc.toISOString(), utc.getTime());
console.log(new Date("2024-02-29T10:00:00Z").toISOString());
console.log(new Date("not a date").getTime(), String(new Date("nope")));

Output

1970-01-01T00:00:00.000Z
2024-01-31T12:30:00.000Z 1706704200000
2024-02-29T10:00:00.000Z
NaN Invalid Date

Reading and doing arithmetic

JavaScript

const d = new Date(Date.UTC(2024, 0, 31));
console.log(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate(), d.getUTCDay());
d.setUTCMonth(d.getUTCMonth() + 1);        // Jan 31 + 1 month → overflows!
console.log(d.toISOString().slice(0, 10));

const a = new Date("2024-03-01T00:00:00Z"), b = new Date("2024-03-15T00:00:00Z");
const days = (b - a) / (1000 * 60 * 60 * 24);   // subtraction → milliseconds
console.log(days, a < b);
const addDays = (date, n) => new Date(date.getTime() + n * 86_400_000);
console.log(addDays(a, 10).toISOString());

Output

2024 0 31 3
2024-03-02
14 true
2024-03-11T00:00:00.000Z

Formatting with Intl

JavaScript

const when = new Date(Date.UTC(2024, 6, 4, 15, 5));
const opts = { dateStyle: "long", timeStyle: "short", timeZone: "UTC" };
console.log(new Intl.DateTimeFormat("en-US", opts).format(when));
console.log(new Intl.DateTimeFormat("en-GB", opts).format(when));
console.log(when.toLocaleDateString("en-IN", { timeZone: "Asia/Kolkata", weekday: "short", day: "numeric", month: "short" }));

console.log(new Intl.NumberFormat("en-IN", { style: "currency", currency: "INR" }).format(1234567.5));
console.log(new Intl.NumberFormat("en-US", { notation: "compact" }).format(2_500_000));
console.log(new Intl.NumberFormat("en-US", { style: "percent" }).format(0.256));
const rtf = new Intl.RelativeTimeFormat("en", { numeric: "auto" });
console.log(rtf.format(-1, "day"), "|", rtf.format(3, "week"));
console.log(new Intl.ListFormat("en", { type: "conjunction" }).format(["JS", "CSS", "HTML"]));
console.log(new Intl.PluralRules("en", { type: "ordinal" }).select(22));

Output

July 4, 2024 at 3:05 PM
4 July 2024 at 15:05
Thu, 4 Jul
₹12,34,567.50
2.5M
26%
yesterday | in 3 weeks
JS, CSS, and HTML
two

Temporal (the future of dates)

Temporal (Stage 3, shipping in browsers from 2025–26) replaces Date with immutable, zone-aware types: Temporal.PlainDate, PlainTime, ZonedDateTime, Instant, Duration. Month numbers start at 1, and arithmetic like date.add({ months: 1 }) is calendar-correct.

JavaScript

// Temporal isn't enabled by default in Node 24 — shown for reference.
const d = Temporal.PlainDate.from("2024-01-31");
console.log(d.add({ months: 1 }).toString());   // 2024-02-29 (clamped, not overflowed)

How do you compare two dates?