Dates

Date is famously awkward; know its traps and reach for the modern option when possible.

new Date();                       // now
new Date("2025-01-31T10:00:00Z"); // ISO parse (safe)
new Date(2025, 0, 31);            // ⚠️ month is 0-indexed! → January
Date.now();                       // ms since epoch (number)

Getting & formatting#

const d = new Date();
d.getFullYear(); d.getMonth() /*0-11*/; d.getDate() /*1-31*/;
d.getDay();      // 0=Sunday
d.getHours(); d.getMinutes();
d.getTime();     // epoch ms
d.toISOString(); // "2025-01-31T10:00:00.000Z" (UTC)

⚠️ Traps: months are 0-indexed; getDay is weekday not date; parsing non-ISO strings is implementation-defined; Date is mutable.

Intl formatting 🟢#

new Intl.DateTimeFormat("en-IN", {
  dateStyle: "long", timeStyle: "short", timeZone: "Asia/Kolkata"
}).format(new Date());
// e.g. "31 January 2025 at 3:30 pm"

new Intl.RelativeTimeFormat("en").format(-1, "day"); // "yesterday"
new Intl.NumberFormat("en-IN", { style: "currency", currency: "INR" }).format(1234.5);
// "₹1,234.50"

Temporal (the future) 🟢#

The Temporal proposal fixes Date's design: immutable objects, explicit time zones, no 0-indexed months.

// Temporal.Now.plainDateISO();      → 2025-01-31
// Temporal.PlainDate.from("2025-01-31").add({ days: 5 });

Not yet universal at time of writing — use a polyfill, or a library (Luxon, date-fns, Day.js) for production date math today.

Interview Q&A#

Q1. Why is new Date(2025, 1, 1) February, not January? The month argument in the numeric Date constructor is 0-indexed, so 0 is January and 1 is February. Only the month is 0-based — the day argument is 1-based, which makes it an easy trap.

Q2. How do you format a date for a specific locale and time zone? Use Intl.DateTimeFormat with a locale and options including timeZone, e.g. new Intl.DateTimeFormat("en-IN", { dateStyle: "long", timeStyle: "short", timeZone: "Asia/Kolkata" }).format(new Date()). It handles locale-aware ordering, month names, and zone conversion without manual math.

Q3. What does Temporal improve over Date? Temporal offers immutable objects (no accidental mutation), first-class explicit time-zone and calendar support, non-ambiguous parsing, and 1-indexed months — fixing Date's core design flaws. Until it's universal, use a polyfill or a library like Luxon or date-fns for production date math.