Native Relative Time Formatting In Typescript/Javascript

timeUtils.ts

export function getRelativeTime(
  date: Date | string | number,
  locale: string = navigator.language
): string {
  // normalize to a Date, and bail out if it’s invalid
  const target =
    typeof date === 'string' || typeof date === 'number'
      ? new Date(date)
      : date;
  if (isNaN(target.getTime())) {
    console.warn(`getRelativeTime: invalid date input ${date}`);
    return '';
  }

  // compute difference in seconds
  const now = Date.now();
  const diffSec = Math.round((target.getTime() - now) / 1000);

  // table of thresholds, the corresponding unit, and how many seconds each unit represents
  const intervals: Array<{
    limit: number;
    unit: Intl.RelativeTimeFormatUnit;
    secsPerUnit: number;
  }> = [
    { limit: 60, unit: 'second', secsPerUnit: 1 },
    { limit: 3600, unit: 'minute', secsPerUnit: 60 },
    { limit: 86400, unit: 'hour', secsPerUnit: 3600 },
    { limit: 604800, unit: 'day', secsPerUnit: 86400 },
    { limit: 2592000, unit: 'week', secsPerUnit: 604800 },
    { limit: 31536000, unit: 'month', secsPerUnit: 2592000 },
    { limit: Infinity, unit: 'year', secsPerUnit: 31536000 },
  ];

  // find the first interval where |diffSec| < limit
  const { unit, secsPerUnit } = intervals.find(
    (i) => Math.abs(diffSec) < i.limit
  )!;

  // compute how many of those units
  const value = Math.round(diffSec / secsPerUnit);

  // format via Intl.RelativeTimeFormat
  const rtf = new Intl.RelativeTimeFormat(locale, {
    numeric: 'auto',
    style: 'long',
  });
  return rtf.format(value, unit);
}