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JavaScript Date & Intl API - The Complete Interview Masterclass (0-100)

This directory contains a comprehensive guide to mastering Date and Time in JavaScript, ranging from fundamental concepts to expert-level internationalization and edge-case handling.

📁 Directory Structure

🛠️ Core Date API (The Foundation)

  • 01-Creation-and-Epoch.js: Epoch time, the 4 ways to create dates, and Timestamp vs. Timezone.
  • 02-Getters-Setters-Gotchas.js: Essential methods, the 0-indexed month trap, and validation techniques.
  • 03-Date-Arithmetic-Logic.js: Adding/subtracting time, leap year logic, and best practices for storage vs. rendering.
  • 04-Expert-Edge-Cases.js: Month overflow bugs, high-precision timing, business day calculation, and the future (Temporal API).

🌐 Internationalization (Intl API)

Located in the /Intl sub-directory:

  • 01-Formatting-Basics.js: Locales and the shortcut dateStyle/timeStyle APIs.
  • 02-Advanced-Options.js: Fine-grained control, formatToParts, and custom calendars.
  • 03-TimeZones.js: IANA timezone names, DST handling, and offset calculation.
  • 04-Range-and-Relative.js: formatRange and Intl.RelativeTimeFormat (e.g., "2 days ago").
  • 05-Interview-Questions.js: Targeted challenges and common pitfalls.
  • 06-Expert-Level-Intl.js: Millisecond precision, day periods, and resolvedOptions() mastery.

🏆 Challenges

  • Complete-Date-Challenges.js: 20 hand-picked interview tasks covering every level from beginner to expert.

🚀 Key Takeaways for Interview Mastery

1. The "Month" Trap

In JavaScript, months are 0-indexed (January is 0, December is 11). However, days of the month are 1-indexed. Tip: new Date(2026, 2, 0) gives you the last day of February!

2. Storage vs. Display

  • Rule: Always Store in UTC (Timestamps or ISO-8601 strings) and Display in Local time using Intl.DateTimeFormat.

3. Timestamp vs. Timezone

  • A Timestamp is a single point in time globally (UTC).
  • A Timezone is just a "lens" through which we view that point.

4. Why Intl over Date?

  • Intl handles Daylight Saving Time (DST) and locale-specific formatting (like Arabic or Japanese calendars) automatically.
  • Intl.DateTimeFormat instances should be cached for performance if used in loops.

5. DST Transitions

Dates are tricky during "Spring Forward" (an hour disappears) and "Fall Back" (an hour happens twice). Use setDate(getDate() + n) for human-safe arithmetic instead of raw millisecond math.

⚡ Quick Snippets for the Interview

Create Local Midnight Date

const [y, m, d] = '2026-03-15'.split('-').map(Number);
const localDate = new Date(y, m - 1, d);

Format for Indian User (24h)

new Intl.DateTimeFormat('en-IN', {
  timeStyle: 'medium',
  hourCycle: 'h23',
  timeZone: 'Asia/Kolkata',
}).format(new Date());

Relative Time ("Yesterday")

const rtf = new Intl.RelativeTimeFormat('en', { numeric: 'auto' });
rtf.format(-1, 'day'); // "yesterday"

Range Formatting

const fmt = new Intl.DateTimeFormat('en-US', { dateStyle: 'medium' });
fmt.formatRange(new Date(), new Date(Date.now() + 86400000)); // "Oct 25 – 26, 2023"