getTimeSince - function for getting a human-readable value of the time since/between datetimes

getTimeSince.js

const ordinalizeNumber = n => {
  const rule = new Intl.PluralRules('en-US', { type: 'ordinal' }).select(n);
  const suffix = ({
    one: 'st',
    two: 'nd',
    few: 'rd',
    other: 'th',
  })[rule];
  return `${n}${suffix}`;
}

const getTimeSince = (_fromDate, _toDate, dateStringCap) => {
  const throwError = () => {
    throw new Error('getTimeSince requires 1-2 arguments, of type date or date-string');
  };
  if (typeof _fromDate === 'undefined' || isNaN(Number(_fromDate)) || (typeof _toDate !== 'undefined' && isNaN(Number(_toDate)))) throwError();
  const toPresent = _toDate === undefined || Math.abs(Number(_toDate) - Number(new Date())) < 50;
  const fromDate = Number(new Date(_fromDate));
  const toDate = Number(_toDate === undefined ? new Date() : new Date(_toDate ?? null));
  if (isNaN(fromDate) || isNaN(toDate)) throwError();
  let formatTimeSince = new Intl.RelativeTimeFormat('en', {
    localeMatcher: 'best fit',
    style: 'long',
    numeric: 'auto',
  });
  let diff = fromDate - toDate;
  if (!isNaN(dateStringCap) && Math.abs(diff) >= Number(dateStringCap)) {
    const [weekday, month, dateNumStr, year] = (
      new Date(fromDate).toLocaleString('en-us', {
        weekday: 'short',
        month: 'short',
        day: 'numeric',
        year: 'numeric',
      }).replaceAll(',', '').split(' ')
    );
    return `${weekday} ${month} ${ordinalizeNumber(Number(dateNumStr))}, ${year}`;
  }
  if (Math.abs(diff) < 1000) return toPresent ? 'now' : 'simultaneously';
  for (const [unit, value] of [
    ['year',   1000 * 60 * 60 * 24 * 365],
    ['month',  1000 * 60 * 60 * 24 * 31],
    ['week',   1000 * 60 * 60 * 24 * 7],
    ['day',    1000 * 60 * 60 * 24],
    ['hour',   1000 * 60 * 60],
    ['minute', 1000 * 60],
    ['second', 1000],
  ]) if (Math.abs(diff) >= value) {
    const { sign, floor, ceil } = Math;
    let result = formatTimeSince.format(
      (sign(diff) === 1 ? floor : ceil)(diff / value),
      unit
  	);
    if (!toPresent) result = Math.sign(diff) === 1
      ? result.replace('in ', '') + ' later'
      : result.replace('ago', 'prior');
    return result;
  }
}

Great script. One suggestion: .replace(/,/g, '') instead of the replaceAll will support more targets