Dates are where confident developers get humbled. The essentials plus the traps:
Creating and reading
const now = new Date();
const d = new Date("2026-07-04"); // ISO string — the safe format
const d2 = new Date(2026, 6, 4); // ⚠️ month is 0-INDEXED: 6 = July!
d.getFullYear() // 2026
d.getMonth() // 6 ⚠️ +1 for humans
d.getDate() // 4 (day of month)
d.getDay() // 6 (weekday, 0=Sunday)
Date.now() // timestamp ms — for measuring/IDsFormatting — stop concatenating, use Intl
new Intl.DateTimeFormat("en-IN", { dateStyle: "medium" }).format(d)
// "4 Jul 2026" — locale-correct, zero libraries
d.toLocaleDateString("en-IN", { weekday: "long", day: "numeric", month: "long" })
// "Saturday, 4 July 2026"
new Intl.RelativeTimeFormat("en").format(-2, "day") // "2 days ago"Date math
const tomorrow = new Date(now); tomorrow.setDate(now.getDate() + 1); // handles month/year rollover! const days = Math.round((examDate - now) / 86400000); // countdown
Traps: getMonth() zero-indexing, timezone shifts when parsing non-ISO strings, and mutating setters. For heavy date work, the upcoming Temporal API fixes all of it — until then, ISO strings + Intl.