JavaScript Date & Time Cheat Sheet: Date, Intl, Timezones & Formatting

JavaScript’s Date stores an instant (milliseconds since Unix epoch) and prints in the local timezone unless you use UTC methods or Intl. Parse ISO-8601 (2026-09-07T09:00:00+05:00) carefully: date-only "2026-09-07" is UTC midnight and can shift the calendar day. Prefer Intl.DateTimeFormat for display.

This JavaScript cheat sheet is written for people searching for a fast, accurate reference: students, junior developers, and teams shipping production software. Pin it, copy the snippets, and come back when syntax slips.

Quick reference

• Now: Date.now() ms epoch. new Date() current instant.
• ISO: date.toISOString() always UTC with Z. Good for APIs.
• Parse: new Date(iso) for full ISO. Avoid new Date("09/07/2026") — US vs local parsing.
• Get: getFullYear, getMonth (0–11), getDate (day of month). UTC variants exist.
• Intl: new Intl.DateTimeFormat("en-PK", { dateStyle: "medium", timeZone: "Asia/Karachi" }).
• Compare: Compare timestamps or ISO strings, not formatted display text.
• Add days: Work in ms (+ 86400000) or Temporal where available — DST makes “add 1 day” tricky.
• Temporal: The newer Temporal API (where supported) models calendar dates separately from instants.

Copy-paste examples

Format in a known timezone

Do not split toISOString() and treat the UTC date as the user’s calendar day.

function formatKarachi(iso) {
  return new Intl.DateTimeFormat('en-GB', {
    dateStyle: 'medium',
    timeStyle: 'short',
    timeZone: 'Asia/Karachi',
  }).format(new Date(iso));
}
function isSameLocalDay(a, b, timeZone) {
  const fmt = new Intl.DateTimeFormat('en-CA', { timeZone, dateStyle: 'short' });
  return fmt.format(a) === fmt.format(b);
}

Common mistakes

• getMonth() displayed without +1 so January is 0.
• Parsing "YYYY-MM-DD" and showing the previous evening in US timezones.
• Storing display strings ("07/09/2026") in JSON instead of ISO instants.
• Using setHours(0,0,0,0) for “today” in a server TZ that is not the user’s TZ.

FAQ

1. Should I use Moment.js? No for new code. Use Intl and Date, or Temporal / date-fns / luxon if you need a library.

1. How do I send dates to PHP? ISO-8601 UTC (toISOString()) or a Unix timestamp. Parse explicitly on the server.

1. Why is my countdown off by an hour? DST. Adding 24 hours is not always adding a calendar day. Use timezone-aware libraries or Temporal for calendaring.

Related JavaScript cheat sheets

• Javascript Cheat Sheet 2026
• Javascript Array Methods Cheat Sheet
• Javascript String Methods Cheat Sheet

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