frontend/js/common/datetime/relative_time.js

/** Calculate a string representing a date object as relative time from now.
 *
 * Example output: 5 minutes ago, in 3 days.
*/
const tests = [
  60, 'second',
  60, 'minute',
  24, 'hour',
  7, 'day',
];

export default function relativeTime(dateObj) {
  let delta = Math.abs((new Date() - dateObj) / 1000);
  const format = delta >= 0 ? '%s ago' : 'in %s';

  for (let i = 0; i < tests.length; i += 2) {
    if (delta < tests[i]) {
      delta = Math.floor(delta);
      return format.replace(
        '%s',
        delta === 1 ? '1 ' + tests[i + 1] : delta + ' ' + tests[i + 1] + 's'
      );
    }

    delta /= tests[i];
  }

  delta = Math.floor(delta);
  return format.replace('%s', delta === 1 ? '1 week' : delta + ' weeks');
}