Express past and future dates in relative words in JavaScript

to-relative-time.js

/**
 * A lighter version based on https://github.com/jherdman/javascript-relative-time-helpers
 * 
 * Takes a date either in the future or the past and returns distance of time in words from that date up to now
 * 
 * Examples:
 * 
 * - current date: 
 *   toRelativeTime(new Date())
 *   > 'Just now'
 * 
 * - past date
 *   toRelativeTime(new Date('Tue, 21 Jan 2016'))
 *   > '3 days ago'
 * 
 * - future date
 *   toRelativeTime(new Date('Tue, 29 Jan 2016 02:08:09 UTC +00:00'))
 *   > 'in 5 days'
 * 
 */
function toRelativeTime(date) {
  var CONVERSIONS = {
    millisecond: 1, // ms    -> ms
    second: 1000,   // ms    -> sec
    minute: 60,     // sec   -> min
    hour:   60,     // min   -> hour
    day:    24,     // hour  -> day
    month:  30,     // day   -> month (roughly)
    year:   12      // month -> year
  };

  var now = new Date();
  var delta = now - date;
  var future = (delta <= 0);
  delta = Math.abs(delta);

  // default threshold is 0
  if (delta <= 0) { return 'Just now'; }

  var units = null;
  for (var key in CONVERSIONS) {
    if (delta < CONVERSIONS[key])
      break;
    units = key; // keeps track of the selected key over the iteration
    delta = delta / CONVERSIONS[key];
  }
  delta = Math.round(delta);

  // pluralize a unit when the difference is greater than 1.
  if(delta > 1) units += "s";

  return (future ? ["in", delta, units] : [delta, units, "ago"]).join(" ");
}