Time ago in words should be calculated on the client side. http://rails-bestpractices.com/posts/2012/02/10/not-use-time_ago_in_words/

application.js

import DateHelper from './modules/dateHelper';

const dataInit = new DateHelper();
const _timeAgoElems = document.querySelectorAll('.time-ago-in-words');

[..._timeAgoElems].forEach(elem => {
  const time = elem.querySelector('time');
  time.textContent = dataInit.timeAgoInWords(time.dataset.time);
});

dateHelper.js

// helper to work with date object
export default class DateHelper {
  constructor() {
    this.timeAgo = {
      LESS_THAN_MINUTE: 0,
      MINUTE: 1,
      THREE_QUARTERS: 45,
      HOUR: 60,
      HOUR_AND_A_HALF: 90,
      DAY: 1440,
      TWO_DAYS: 2880,
      MONTH: 43200,
      TWO_MONTHS: 86400,
      YEAR: 525600,
      TWO_YEARS: 1051200,
    };
  }
  // get formated time ago in words
  timeAgoInWords(from) {
    const date = new Date();
    date.setTime(Date.parse(from));
    return this.timeInWords(new Date(), date);
  }
  // return tiem in words as a string
  timeInWords(to, from) {
    const seconds = (to - from) / 1000;
    const minutes = Math.abs(Math.floor(seconds / this.timeAgo.HOUR));
    const tense = seconds < 0 ? ' from now' : ' ago';
    if (minutes === this.timeAgo.LESS_THAN_MINUTE) {
      return `less than a minute${tense}`;
    }
    if (minutes === this.timeAgo.MINUTE) {
      return `a minute${tense}`;
    }
    if (minutes < this.timeAgo.THREE_QUARTERS) {
      return `${minutes} minutes${tense}`;
    }
    if (minutes < this.timeAgo.HOUR_AND_A_HALF) {
      return `about an hour${tense}`;
    }
    if (minutes < this.timeAgo.DAY) {
      return `about ${Math.floor(minutes / this.timeAgo.HOUR)} hours${tense}`;
    }
    if (minutes < this.timeAgo.TWO_DAYS) {
      return `a day${tense}`;
    }
    if (minutes < this.timeAgo.MONTH) {
      return `${Math.floor(minutes / this.timeAgo.DAY)} days${tense}`;
    }
    if (minutes < this.timeAgo.TWO_MONTHS) {
      return `about a month${tense}`;
    }
    if (minutes < this.timeAgo.YEAR) {
      return `${Math.floor(minutes / this.timeAgo.MONTH)} months${tense}`;
    }
    if (minutes < this.timeAgo.TWO_YEARS) {
      return `about a year${tense}`;
    }
    return `over ${Math.floor(minutes / this.timeAgo.YEAR)} years`;
  }
}

idex.html.erb

<div id="posts">
  <% @posts.each do |post| %>
    <div class="post-<%= post.id %>">
      <p class="date time-ago-in-words">
        Published <time data-time="<%= post.created_at.to_s %>"></time> by <%= post.user_email %>
      </p>
    </div>
  <% end %>
</div>