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"]
}

this is a compromise for space. it will show seconds up to 119, and then 2 minutes. this avoids renderings that are incorrect, like "1 minutes ago", and awkward, like "1 minute(s) ago". 135 bytes is too short to handle singular/plural...

Hi Jed,

I'd prefer adding the singular/plurar, which adds just a couple of bytes, but from an aesthetic pov it seems worth it? No? (This also fixes the 60seconds vs. 1minute thing as mentioned by @tsenart)

andy

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 >= b[c]; // while the time left exceeds the current unit,
a /= b[c++] // divide the time by the unit and increment.
);
return ~ a + // the rounded remaining unit + // <- had to put a space between the two tildes to avoid display-problems in this text field
" " + // 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, +
( ~a > 1 ? "s ": "") + //adding singular/plurar is worth the couple of bytes
" ago" // and "ago"
}

//Please note I had to put a space between the rounding operators, since they triggered a strike-through

OK - see a slight flaw, if the time < 2 ms, but I could live with that ;-)

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

well done!

How about Years?
Aeons? Ha...