JavaScript Dates: Creating, Formatting, Comparing, and Manipulating Time

const now = new Date();
console.log(now);
// Thu Sep 19 2026 14:32:07 GMT+0530 (your current moment)

const diwali = new Date("2026-11-08");
// ISO format — the reliable one ✅const meeting = new Date("2026-11-08T14:30:00");  // with time attached

// new Date(year, month, day, hours, minutes, seconds)const birthday = new Date(2000, 5, 15, 10, 30);

const epoch = new Date(0);
// Jan 1, 1970 — time zero 🕛const someDay = new Date(1700000000000); // a moment 1.7 trillion ms later

new Date(2000, 5, 15);
// May 15 — NOT June!

const june15 = new Date(2000, 5, 15);
// ✅ June 15 (month 5 = June)

const date = new Date("2026-09-19T14:30:45");
console.log(date.getFullYear());
// 2026 — the yearconsole.log(date.getMonth());      // 8 — September (zero-indexed! 🚨)console.log(date.getDate());       // 19 — day of the MONTHconsole.log(date.getDay());        // 6 — day of the WEEK (0 = Sunday!)console.log(date.getHours());      // 14console.log(date.getMinutes());    // 30console.log(date.getSeconds());    // 45console.log(date.getTime());       // 1789… — milliseconds since epoch

const d = new Date("2026-09-19");
// a Saturday
console.log(d.getDate());
// 19 — the 19th of Septemberconsole.log(d.getDay());    // 6  — Saturday (Sunday=0, Mon=1, … Sat=6)

const event = new Date("2026-09-19T10:00:00");
event.setFullYear(2027);
event.setMonth(11);
// December (0-indexed!)event.setDate(25);event.setHours(18);
console.log(event.toDateString());
// "Fri Dec 25 2027" 🎄

const d = new Date("2026-01-31");
d.setMonth(1);
// Feb 31 doesn't exist…console.log(d.toDateString());  // rolls into early March — handled!
// This makes date math beautifully easy:const today = new Date("2026-09-19");today.setDate(today.getDate() + 30);    // "30 days from now" ✅console.log(today.toDateString());      // "Mon Oct 19 2026" — month rollover automatic!

const now = new Date("2026-09-19T14:30:00");
// Local date string:console.log(now.toLocaleDateString("en-IN"));   // "19/9/2026" 🇮🇳console.log(now.toLocaleDateString("en-US"));   // "9/19/2026" 🇺🇸console.log(now.toLocaleDateString("hi-IN"));   // "19/9/2026" (Hindi digits style)
// Local time string:console.log(now.toLocaleTimeString("en-IN"));   // "2:30:00 pm"
// Both:console.log(now.toLocaleString("en-IN"));       // "19/9/2026, 2:30:00 pm"

const now = new Date("2026-09-19T14:30:00");
console.log(now.toLocaleDateString("en-IN", {
  weekday: "long", // Saturday  year: "numeric",      // 2026  month: "long",        // September  day: "numeric"        // 19}));// "Saturday, 19 September 2026" ✨
  console.log(now.toLocaleTimeString("en-IN", {
    hour: "2-digit", minute: "2-digit", hour12: true
  }));
  // "02:30 pm"

const d = new Date("2026-09-19T14:30:00");
d.toDateString();
// "Sat Sep 19 2026"d.toISOString();       // "2026-09-19T09:00:00.000Z" — for APIs/databases ✅d.toUTCString();       // "Sat, 19 Sep 2026 09:00:00 GMT"

const deadline = new Date("2026-10-01");
const today = new Date("2026-09-19");
console.log(today < deadline);
// true — still time ✅console.log(today > deadline);     // false
// Real-world: is the offer still valid?if (today <= deadline) {  console.log("🎉 Offer is active!");} else {  console.log("😢 Offer expired");}

const a = new Date("2026-09-19");
const b = new Date("2026-09-19");
console.log(a === b);
// false 😱 — different objects!console.log(a.getTime() === b.getTime());   // true ✅ — same moment!

const start = new Date("2026-09-01");
const end = new Date("2026-09-19");
const diffMs = end - start;
// milliseconds betweenconst diffDays = diffMs / (1000 * 60 * 60 * 24);   // ms → s → min → hr → day
console.log(diffDays);
// 18 days

function daysUntil(dateString) {
  const target = new Date(dateString);
  const now = new Date();
  const diffMs = target - now;
  const diffDays = Math.ceil(diffMs /(1000 * 60 * 60 * 24));
  // round UP
  if (diffDays < 0) return `That date passed ${-diffDays} days ago!`;
  if (diffDays === 0) return "It's today! 🎉";
  return `${diffDays} days to go! ⏳`;
}console.log(daysUntil("2026-12-25"));
// "97 days to go! ⏳"
console.log(daysUntil("2026-01-01"));
// "That date passed 261 days ago!"

milliseconds ÷ 1000 → seconds ÷ 60 → minutes ÷ 60 → hours ÷ 24 → days

// 🎂 Age Calculator — dates, comparison, and math together
function calculateAge(birthdateString) {
  const birth = new Date(birthdateString);
  const today = new Date();
  // Basic year difference:
  let age = today.getFullYear() - birth.getFullYear();
  // ⚠️ But has this year's birthday happened yet?
  const birthdayThisYear = new Date( today.getFullYear(), birth.getMonth(), // their birth month
  birth.getDate() // their birth day
);
  if (today < birthdayThisYear) {
    age--;
    // birthday still coming — subtract one ✅
  } return age;
}console.log(calculateAge("2000-06-15"));
// 25 (before June) or 26 (after)
console.log(calculateAge("2000-12-25"));
// correctly handles December birthdays