How to convert milliseconds into human readable form?

Well, since nobody else has stepped up, I'll write the easy code to do this:

x = ms / 1000
seconds = x % 60
x /= 60
minutes = x % 60
x /= 60
hours = x % 24
x /= 24
days = x

I'm just glad you stopped at days and didn't ask for months. :)

Note that in the above, it is assumed that / represents truncating integer division. If you use this code in a language where / represents floating point division, you will need to manually truncate the results of the division as needed.

Let A be the amount of milliseconds. Then you have:

seconds=(A/1000)%60
minutes=(A/(1000*60))%60
hours=(A/(1000*60*60))%24

and so on (% is the modulus operator).

Hope this helps.

Both solutions below use javascript (I had no idea the solution was language agnostic!). Both solutions will need to be extended if capturing durations > 1 month.

Solution 1: Use the Date object

var date = new Date(536643021);
var str = '';
str += date.getUTCDate()-1 + " days, ";
str += date.getUTCHours() + " hours, ";
str += date.getUTCMinutes() + " minutes, ";
str += date.getUTCSeconds() + " seconds, ";
str += date.getUTCMilliseconds() + " millis";
console.log(str);

Gives:

"6 days, 5 hours, 4 minutes, 3 seconds, 21 millis"

Libraries are helpful, but why use a library when you can re-invent the wheel! :)

Solution 2: Write your own parser

var getDuration = function(millis){
    var dur = {};
    var units = [
        {label:"millis",    mod:1000},
        {label:"seconds",   mod:60},
        {label:"minutes",   mod:60},
        {label:"hours",     mod:24},
        {label:"days",      mod:31}
    ];
    // calculate the individual unit values...
    units.forEach(function(u){
        millis = (millis - (dur[u.label] = (millis % u.mod))) / u.mod;
    });
    // convert object to a string representation...
    var nonZero = function(u){ return dur[u.label]; };
    dur.toString = function(){
        return units
            .reverse()
            .filter(nonZero)
            .map(function(u){
                return dur[u.label] + " " + (dur[u.label]==1?u.label.slice(0,-1):u.label);
            })
            .join(', ');
    };
    return dur;
};

Creates a "duration" object, with whatever fields you require. Formatting a timestamp then becomes simple...

console.log(getDuration(536643021).toString());

Gives:

"6 days, 5 hours, 4 minutes, 3 seconds, 21 millis"