Subtracts a date from the current time, and presents it (like Twitter) as "2s" or "2m" or "2h" ago, or as a date.

getTimeAgo.js

var getTimeAgo = function(datestring) {
  // Takes in any datestring that can be used with the Date() constructor:
  // e.g., "10/22/15" or "Wed Nov 29 2015 13:42:33 GMT-0800 (PST)"
  // and outputs a date like Twitter's datetimestamps. 
  var d1 = new Date();
  var d2 = new Date(datestring);
  var timeAgo = d1.getTime()-d2.getTime(); 
  if ((timeAgo/1000/60/60/24) >= 1) {
    // return full date: "Nov 10" if same year, or "10 Nov 2015"
    var months = ["Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"];
    if (d2.getFullYear() !== d1.getFullYear()) {
      return d2.getDate() + ' ' + months[d2.getMonth()] + ' ' + d2.getFullYear().toString().substr(2,2);
    } else {
      return months[d2.getMonth()] + ' ' + d2.getDate();
    }
  } else if ((timeAgo/1000/60/60) >= 1) {
    // return hours if less than 24: "2h"
    return ""+Math.floor(timeAgo/1000/60/60)+"h";
  } else if ((timeAgo/1000/60) >= 1) {
    // return minutes if less than 60: "17m"
    return ""+Math.floor(timeAgo/1000/60)+"m";
  } else if ((timeAgo/1000) >= 1) {
    // return seconds if less than 60: "42s"
    return ""+Math.floor(timeAgo/1000)+"s";
  } else {
    // otherwise, let's just say it was "now"
    return "now";
  }
}