Convert time interval given in seconds into more human readable form

 function secondsToString(seconds)
{
var numyears = Math.floor(seconds / 31536000);
var numdays = Math.floor((seconds % 31536000) / 86400); 
var numhours = Math.floor(((seconds % 31536000) % 86400) / 3600);
var numminutes = Math.floor((((seconds % 31536000) % 86400) % 3600) / 60);
var numseconds = (((seconds % 31536000) % 86400) % 3600) % 60;
return numyears + " years " +  numdays + " days " + numhours + " hours " + numminutes + " minutes " + numseconds + " seconds";

}

With help of Royi we've got code that outputs time interval in a human readable form:

function millisecondsToStr (milliseconds) {
    // TIP: to find current time in milliseconds, use:
    // var  current_time_milliseconds = new Date().getTime();

    function numberEnding (number) {
        return (number > 1) ? 's' : '';
    }

    var temp = Math.floor(milliseconds / 1000);
    var years = Math.floor(temp / 31536000);
    if (years) {
        return years + ' year' + numberEnding(years);
    }
    //TODO: Months! Maybe weeks? 
    var days = Math.floor((temp %= 31536000) / 86400);
    if (days) {
        return days + ' day' + numberEnding(days);
    }
    var hours = Math.floor((temp %= 86400) / 3600);
    if (hours) {
        return hours + ' hour' + numberEnding(hours);
    }
    var minutes = Math.floor((temp %= 3600) / 60);
    if (minutes) {
        return minutes + ' minute' + numberEnding(minutes);
    }
    var seconds = temp % 60;
    if (seconds) {
        return seconds + ' second' + numberEnding(seconds);
    }
    return 'less than a second'; //'just now' //or other string you like;
}

If you are interested in an existing javascript library that does the job very well, you may want to check moment.js.

More specifically, the relevant moment.js piece for your question is durations.

Here are some examples of how you can take advantage of it to achieve your task:

var duration = moment.duration(31536000);

// Using the built-in humanize function:
console.log(duration.humanize());   // Output: "9 hours"
console.log(duration.humanize(true));   // Output: "in 9 hours"

moment.js has built-in support for 50+ human languages, so if you use the humanize() method you get multi-language support for free.

If you want to display the exact time information, you can take advantage of the moment-precise-range plug-in for moment.js that was created exactly for this purpose:

console.log(moment.preciseDiff(0, 39240754000);
// Output: 1 year 2 months 30 days 5 hours 12 minutes 34 seconds

One thing to note is that currently moment.js does not support weeks / days (in week) for duration object.

Hope this helps!