Greasemonkey script that changes gmail's timestamps to relative time in basic html view

gmail-agecounter.user.js

'use strict';
// ==UserScript==
// @name        GMail Age-Counter
// @namespace   http://nkmathew.net
// @description Describes time elapsed since an email message was received
// @require     https://ajax.googleapis.com/ajax/libs/jquery/2.1.4/jquery.min.js
// @require     https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.12.0/moment.min.js
// @include     https://mail.google.com/mail/u/0/h/*
// @version     0.1
// @grant       none
// ==/UserScript==

/**
 * Works in Basic GMail only
 */
(function () {
  window.addEventListener('load', function () {
    function pluralize(number, text) {
      if (number == 1) {
        return number + ' ' + text;
      } else {
        return number + ' ' + text + 's';
      }
    }

    function describeDateTime(date_time, fmt) {
      var val = moment().diff(moment(date_time, fmt));
      var msg = '';
      val = moment.duration(val)['_data'];
      if (val.years && !val.months) {
        msg = pluralize(val.years, 'year');
      } else if (val.years && val.months) {
        msg = pluralize(val.years, 'year') + ', ' + pluralize(val.months, 'month');
      } else if (val.months && !val.days) {
        msg = pluralize(val.months, 'month');
      } else if (val.months && val.days) {
        msg = pluralize(val.months, 'month') + ', ' + pluralize(val.days, 'day');
      } else if (val.days && !val.hours) {
        msg = pluralize(val.days, 'day');
      } else if (val.days && val.hours) {
        msg = pluralize(val.days, 'day') + ', ' + pluralize(val.hours, 'hour');
      } else if (val.hours && !val.minutes) {
        msg = pluralize(val.hours, 'hour');
      } else if (val.hours && val.minutes) {
        msg = pluralize(val.hours, 'hour') + ', ' + pluralize(val.minutes, 'minute');
      } else if (val.minutes) {
        msg = pluralize(val.minutes, 'minute')
      } else {
        if (!val.seconds) {
          msg = 'a second';
        } else {
          msg = val.seconds + ' seconds';
        }
      }
      return 'Sent ' + msg + ' ago';
    }

    const selector = 'td > table > tbody > tr > td > table > tbody > tr > td > table > tbody > tr > td[align="right"]';
    const fmt = 'ddd MMM D yyyy HH:mm A';
    $(selector).each(function() {
      var currDate = this.textContent;
      const currHTML = this.innerHTML.replace(currDate, '');
      currDate = currDate.replace(' at', '');
      var datetime = moment(currDate, fmt).format();
      const newHTML = '<time is="relative-time" class="agecounter" datetime="' + datetime + '" title="' + currDate + '"> ' + describeDateTime(currDate, fmt) + '</time>';
      $(this).html(currHTML + newHTML);
      $('.agecounter').css({
        'font-family': 'Helvetica,Arial'
      });
    });
  }, false);
})();