$ snippet format-date

Format a date without a library

YYYY-MM-DD and ISO-style output, plus a 'time ago' helper, using only built-in APIs.

JavaScript

/** "2026-08-28" in local time (toISOString() would give UTC). */
function toYMD(date = new Date()) {
  const p = (n) => String(n).padStart(2, "0");
  return `${date.getFullYear()}-${p(date.getMonth() + 1)}-${p(date.getDate())}`;
}

/** "2026-08-28 14:05" */
function toYMDHM(date = new Date()) {
  const p = (n) => String(n).padStart(2, "0");
  return `${toYMD(date)} ${p(date.getHours())}:${p(date.getMinutes())}`;
}

/** "3 hours ago", "in 2 days" — localized via Intl.RelativeTimeFormat. */
function timeAgo(date, locale = "en") {
  const rtf = new Intl.RelativeTimeFormat(locale, { numeric: "auto" });
  const diff = (date.getTime() - Date.now()) / 1000;
  const units = [["year", 31536000], ["month", 2592000], ["week", 604800],
                 ["day", 86400], ["hour", 3600], ["minute", 60], ["second", 1]];
  for (const [unit, secs] of units) {
    if (Math.abs(diff) >= secs || unit === "second")
      return rtf.format(Math.round(diff / secs), unit);
  }
}

How it works

The most common bug is calling toISOString().slice(0, 10) for a date string: it converts to UTC first, so late-evening dates shift to tomorrow for users west of Greenwich. Build the string from the local getters instead. For human-friendly relative times, Intl.RelativeTimeFormat handles pluralization and language for you.

Gotchas

getMonth() is zero-based; getDate() is the day of the month, getDay() is the weekday.
For full locale-aware formatting ("28 Aug 2026", "28/08/2026") use Intl.DateTimeFormat rather than string-building.
Parsing new Date("2026-08-28") treats the string as UTC midnight, while new Date("2026-08-28T00:00") is local midnight. Include a time to get local.

Try it

Result

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