Language-sensitive relative time formatting in JavaScript.

language-sensitive-relative-time-formatting.js

/**
 * Language-sensitive relative time formatting.
 * @see https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Intl/RelativeTimeFormat
 * @example
 * // now
 * console.log(formatRelativeTime(
 *     '2000/01/01 00:00:00',
 *     '2000/01/01 00:00:00'
 * ));
 *
 * // next month
 * console.log(formatRelativeTime(
 *     '2000/02/01 00:00:00',
 *     '2000/01/01 00:00:00'
 * ));
 *
 * // 2 years ago
 * console.log(formatRelativeTime(
 *     '2000/01/01 00:00:00',
 *     '2001/01/01 00:00:00'
 * ));
 */
function formatRelativeTime(ts1, ts2, fmt)
{
    ts1 = (new Date(ts1)).getTime();
    ts2 = ts2 ? (new Date(ts2)).getTime() : Date.now();

    const elapsed = Math.round((ts1 - ts2) / 1000);
    const difference = Math.abs(elapsed);

    fmt = { numeric: 'auto', ...fmt };

    fmt.units ??= {
        year: 31536000,
        month: 2629800,
        week: 604800,
        day: 86400,
        hour: 3600,
        minute: 60
    };

    const rtf = new Intl.RelativeTimeFormat(fmt?.locale || undefined, fmt);

    for (const unit in fmt.units) {
        if (difference >= fmt.units[unit]) {
            return rtf.format(
                Math.floor(elapsed / fmt.units[unit]),
                unit
            );
        }
    }

    return rtf.format(elapsed, 'second');
}

Temporal.ZonedDateTime.from('2025-04-14T10:00[America/New_York]')
  .withTimeZone(Temporal.Now.timeZoneId()) // your time zone
  .toLocaleString(navigator.language);
  
navigator.language; // seems a better choice than ↓
new Intl.DateTimeFormat().resolvedOptions().locale;