format-spec.ts

const SECOND = 1000; // ms
const MINUTE = 60 * SECOND;
const HOUR = 60 * MINUTE;
const DAY = 24 * HOUR;
const WEEK = 7 * DAY;
const MONTH = 4 * WEEK;

export function formatRelativeTimestamp(date: Date): string {
  const agoMs = Date.now() - date.getTime();

  if (agoMs < 30 * SECOND) {
    return 'just now';
  } else if (agoMs < HOUR) {
    const minutes = Math.ceil(agoMs / MINUTE);
    if (minutes > 1) {
      return `${minutes} minutes ago`;
    } else {
      return 'less than a minute ago';
    }
  } else if (agoMs < DAY) {
    const hours = Math.ceil(agoMs / HOUR);
    const plural = (hours > 1) ? 'hours' : 'hour';
    return `${hours} ${plural} ago`;
  } else if (agoMs < WEEK) {
    const days = Math.ceil(agoMs / DAY);
    const plural = (days > 1) ? 'days' : 'day';
    return `${days} ${plural} ago`;
  } else if (agoMs < MONTH) {
    const weeks = Math.ceil(agoMs / WEEK);
    const plural = (weeks > 1) ? 'weeks' : 'week';
    return `${weeks} ${plural} ago`;
  } else {
    const months = Math.ceil(agoMs / MONTH);
    const plural = (months > 1) ? 'months' : 'month';
    return `${months} ${plural} ago`;
  }
}

const obj: { [key: number]: any } = {};

const start = Date.now();

for (let i = 0; i < 10000; i++) {
  obj[i] = formatRelativeTimestamp(new Date());
}

console.log('did in ', Date.now() - start, 'ms');