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

Copyright (c) 2011 Jed Schmidt, http://jed.is
 
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:
 
The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.
 
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

package.json

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