How to convert milliseconds into human readable form?

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:

                            answered 2012-09-14T08:29:25.300
                            
                        

function convertTime(time) {        
    var millis= time % 1000;
    time = parseInt(time/1000);
    var seconds = time % 60;
    time = parseInt(time/60);
    var minutes = time % 60;
    time = parseInt(time/60);
    var hours = time % 24;
    var out = "";
    if(hours && hours > 0) out += hours + " " + ((hours == 1)?"hr":"hrs") + " ";
    if(minutes && minutes > 0) out += minutes + " " + ((minutes == 1)?"min":"mins") + " ";
    if(seconds && seconds > 0) out += seconds + " " + ((seconds == 1)?"sec":"secs") + " ";
    if(millis&& millis> 0) out += millis+ " " + ((millis== 1)?"msec":"msecs") + " ";
    return out.trim();
}

Long serverUptimeSeconds = 
    (System.currentTimeMillis() - SINCE_TIME_IN_MILLISECONDS) / 1000;


String serverUptimeText = 
String.format("%d days %d hours %d minutes %d seconds",
serverUptimeSeconds / 86400,
( serverUptimeSeconds % 86400) / 3600 ,
((serverUptimeSeconds % 86400) % 3600 ) / 60,
((serverUptimeSeconds % 86400) % 3600 ) % 60
);

I'm not able to comment first answer to your question, but there's a small mistake. You should use parseInt or Math.floor to convert floating point numbers to integer, i

var days, hours, minutes, seconds, x;
x = ms / 1000;
seconds = Math.floor(x % 60);
x /= 60;
minutes = Math.floor(x % 60);
x /= 60;
hours = Math.floor(x % 24);
x /= 24;
days = Math.floor(x);

Personally, I use CoffeeScript in my projects and my code looks like that:

getFormattedTime : (ms)->
        x = ms / 1000
        seconds = Math.floor x % 60
        x /= 60
        minutes = Math.floor x % 60
        x /= 60
        hours = Math.floor x % 24
        x /= 24
        days = Math.floor x
        formattedTime = "#{seconds}s"
        if minutes then formattedTime = "#{minutes}m " + formattedTime
        if hours then formattedTime = "#{hours}h " + formattedTime
        formattedTime 

Long expireTime = 69l;
Long tempParam = 0l;

Long seconds = math.mod(expireTime, 60);
tempParam = expireTime - seconds;
expireTime = tempParam/60;
Long minutes = math.mod(expireTime, 60);
tempParam = expireTime - minutes;
expireTime = expireTime/60;
Long hours = math.mod(expireTime, 24);
tempParam = expireTime - hours;
expireTime = expireTime/24;
Long days = math.mod(expireTime, 30);

system.debug(days + '.' + hours + ':' + minutes + ':' + seconds);

This should print: 0.0:1:9