Periodically update the text of `datetime` elements with the relative time elapsed.

updateDateTimes.js

(function (win, doc) {
    'use strict';
    if (!doc.querySelectorAll || !win.Intl || !win.Intl.RelativeTimeFormat) {
        // doesn't cut the mustard.
        return;
    }
    var rtf = new Intl.RelativeTimeFormat('en', {
        localeMatcher: 'best fit',
        numeric: 'always',
        style: 'long'
    });
    var units = {
        'year'      : 1000 * 24 * 60 * 60 * 365,
        'month'     : 1000 * 24 * 60 * 60 * (365/12),
        'week'      : 1000 * 24 * 60 * 60 * 7,
        'day'       : 1000 * 24 * 60 * 60,
        'hour'      : 1000 * 60 * 60,
        'minute'    : 1000 * 60,
        'second'    : 1000
    };
    var unit, then, now, diff, amount, text;
    var updateDatetimes = function () {
        now = Date.now();
        doc.querySelectorAll('time[datetime]').forEach(function(timeElement) {
            text = 'just now';
            then = timeElement.getAttribute('datetime');
            diff = new Date(then) - new Date(now);
            for (unit in units) {
                amount = units[unit];
                if (Math.abs(diff) > amount) {
                    text = rtf.format(Math.round(diff / amount), unit);
                    break;
                }
            }
            timeElement.innerText = text;
        });
    };
    // Run the update every 30 seconds
    setInterval(updateDatetimes, units.second * 30);
}(this, this.document));