Simple "ago" for Date

ago.js

(function(){

  var lang = function(term){
    //todo other languages
    return term;
  }

  Date.prototype.ago = function(){
    var now = new Date();
    var diff = (now - this)/1000;
    if(diff < 60){
      return lang('Just now');
    }
    else if(diff < 120){
      return [Math.floor(diff/60),lang('minute ago')].join(' ');
    }
    else if(diff < 3600){
      return [Math.floor(diff/60),lang('minutes ago')].join(' ');
    }
    else if(diff < 7200){
      return [Math.floor(diff/3600),lang('hour ago')].join(' ');
    }
    else if(diff < 86400){
      return [Math.floor(diff/3600),lang('hours ago')].join(' ');
    }
    else if(diff < 86400*2){
      return lang('Yesterday');
    }
    else if(diff < 86400*4){
      return [Math.floor(diff/86400),lang('days ago')].join(' ');
    }
    return this.toString('yyyy/MM/dd HH:mm');
  }

})();

var testdate = new Date(new Date().getTime()-1000);
console.log(testdate.ago());
var testdate = new Date(new Date().getTime()-60000);
console.log(testdate.ago());
var testdate = new Date(new Date().getTime()-3600000);
console.log(testdate.ago());