Format dates as relative time

// Render a timestamp as "3 days ago" style text with Intl.RelativeTimeFormat.
const STEPS = [
  ['year', 365 * 24 * 60 * 60],
  ['month', 30 * 24 * 60 * 60],
  ['day', 24 * 60 * 60],
  ['hour', 60 * 60],
  ['minute', 60],
  ['second', 1],
];

function timeAgo(date, now = Date.now()) {
  const diff = (new Date(date).getTime() - now) / 1000; // negative means past
  const formatter = new Intl.RelativeTimeFormat('en', { numeric: 'auto' });
  for (const [unit, seconds] of STEPS) {
    if (Math.abs(diff) >= seconds) {
      return formatter.format(Math.round(diff / seconds), unit);
    }
  }
  return formatter.format(0, 'second');
}

console.log(timeAgo(Date.now() - 45 * 60 * 1000)); // 45 minutes ago
console.log(timeAgo(Date.now() + 2 * 24 * 60 * 60 * 1000)); // in 2 days