relative-date.js

function relativeDate(str) {
  var s = ( +new Date() - Date.parse(str) ) / 1e3,
    m = s / 60,
    h = m / 60,
    d = h / 24,
    w = d / 7,
    y = d / 365.242199,
    M = y * 12;
  
  function approx(num) {
    return num < 5 ? 'a few' : Math.round(num);
  };
  
  return s <= 1 ? 'just now'
       : m < 1  ? approx(s) + ' seconds ago'
       
       : m <= 1 ? 'a minute ago'
       : h < 1  ? approx(m) + ' minutes ago'
       
       : h <= 1 ? 'an hour ago'
       : d < 1  ? approx(h) + ' hours ago'
       
       : d <= 1 ? 'yesterday'
       : w < 1  ? approx(d) + ' days ago'
       
       : w <= 1 ? 'last week'
       : M < 1  ? approx(w) + ' weeks ago'
       
       : M <= 1 ? 'last month'
       : y < 1  ? approx(M) + ' months ago'
       
       : y <= 1 ? 'a year ago'
       : approx(y) + ' years ago';
};