Date

JavaScript · Reference cheat sheet

📋 Overview

Date represents an instant in time as milliseconds since the Unix epoch (UTC). Local getters/setters use the host timezone. Prefer ISO 8601 strings and libraries (Temporal, Luxon, dayjs) for calendars and time zones when complexity grows.

🔧 Core concepts

Create: new Date(), new Date(ms), new Date(isoString), Date.UTC(...).
Epoch: Date.now(), date.getTime(), +date.
UTC vs local: getUTC* / setUTC* vs get* / set*.
Format: toISOString(), toLocaleString(locale, options), Intl.DateTimeFormat.
Parse: Date.parse(string) — implementation-defined for non-ISO forms.

const now = new Date();
const utc = Date.UTC(2026, 0, 15); // month 0-based
const fromIso = new Date("2026-07-10T08:00:00.000Z");

💡 Examples

// Add days safely via ms
function addDays(date, days) {
  return new Date(date.getTime() + days * 86_400_000);
}

// Start of local day
function startOfLocalDay(d = new Date()) {
  const x = new Date(d);
  x.setHours(0, 0, 0, 0);
  return x;
}

// Locale format
const fmt = new Intl.DateTimeFormat("en-US", {
  dateStyle: "medium",
  timeStyle: "short",
  timeZone: "America/Los_Angeles",
});
console.log(fmt.format(new Date()));

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

// Diff in whole days (UTC calendar-ish)
function diffDays(a, b) {
  const ms = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate())
    - Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
  return ms / 86_400_000;
}

// Valid check
function isValidDate(d) {
  return d instanceof Date && !Number.isNaN(d.getTime());
}

// Prefer ISO for interchange
JSON.stringify({ at: new Date() }); // {"at":"2026-...Z"} via toJSON

// Manual components (months are 0–11!)
const d = new Date(2026, 6, 10); // Jul 10, 2026 local

⚠️ Pitfalls

Months are 0-based in the Date constructor and setters.
new Date("YYYY-MM-DD") is parsed as UTC; YYYY-MM-DDTHH:mm may be local — prefer full ISO with Z or offset.
Invalid dates have getTime() === NaN.
Mutator methods mutate the instance — clone before changing shared dates.
DST makes “add one day” via setDate safer than fixed hours in local math for calendar days.

number.md — timestamps as numbers
json.md — Date serialization
api.md — Intl and runtime helpers
strings.md — ISO string handling
math.md — rounding durations

Date

Date

📋 Overview

🔧 Core concepts

💡 Examples

⚠️ Pitfalls

🔗 Related

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