calculate # of ms/seconds/minutes/hours/days in the past

annotated.js

function(
  a, // a Date object
  b, // placeholder
  c  // placeholder
){
  for (    // whittle the date down to biggest unit
    b = [  // an array containing the
      1e3, // ms in a second,
      60,  // seconds in a minute,
      60,  // minutes in an hour, and
      24   // hours in a day.
    ],
    a = new Date - a, // get ms since date passed.
    c = 0;            // initialize the cursor.
    a > 2 * b[c];     // while the 2 x time left exceeds the current unit,
    a /= b[c++]       // divide the time by the unit and increment.
  );                  // (note: the 2* above ensures the units are plural!)

  return ~~a +                 // the rounded remaining unit +
    " " +                      // a space + the unit name, which we get by
    "m0second0minute0hour0day" // taking a 0-delimited string of names
     .split(0)                 // splitting it by 0, and
     [c] +                     // taking the final cursor, +
    "s ago"                    // the plural and "ago"
}

index.js

function(a,b,c){for(b=[1e3,60,60,24],a=new Date-a,c=0;a>2*b[c];a/=b[c++]);return~~a+" "+"m0second0minute0hour0day".split(0)[c]+"s ago"}

LICENSE.txt

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
                    Version 2, December 2004

 Copyright (C) 2011 Jed Schmidt <http://jed.is>

 Everyone is permitted to copy and distribute verbatim or modified
 copies of this license document, and changing it is allowed as long
 as the name is changed.

            DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION

  0. You just DO WHAT THE FUCK YOU WANT TO.

package.json

{
  "name": "timeAgo",
  "keywords": ["time", "chronological", "ago"]
}

awesome! this is probably one of my favorite 140bytes.

well done!

How about Years?
Aeons? Ha...