Work with a time span in Javascript

If you're not too worried in accuracy after days, you can simply do the maths

function timeSince(when) { // this ignores months
    var obj = {};
    obj._milliseconds = (new Date()).valueOf() - when.valueOf();
    obj.milliseconds = obj._milliseconds % 1000;
    obj._seconds = (obj._milliseconds - obj.milliseconds) / 1000;
    obj.seconds = obj._seconds % 60;
    obj._minutes = (obj._seconds - obj.seconds) / 60;
    obj.minutes = obj._minutes % 60;
    obj._hours = (obj._minutes - obj.minutes) / 60;
    obj.hours = obj._hours % 24;
    obj._days = (obj._hours - obj.hours) / 24;
    obj.days = obj._days % 365;
    // finally
    obj.years = (obj._days - obj.days) / 365;
    return obj;
}

then timeSince(pastDate); and use the properties as you like.

Otherwise you can use .getUTC* to calculate it, but note it may be slightly slower to calculate

function timeSince(then) {
    var now = new Date(), obj = {};
    obj.milliseconds = now.getUTCMilliseconds() - then.getUTCMilliseconds();
    obj.seconds = now.getUTCSeconds() - then.getUTCSeconds();
    obj.minutes = now.getUTCMinutes() - then.getUTCMinutes();
    obj.hours = now.getUTCHours() - then.getUTCHours();
    obj.days = now.getUTCDate() - then.getUTCDate();
    obj.months = now.getUTCMonth() - then.getUTCMonth();
    obj.years = now.getUTCFullYear() - then.getUTCFullYear();
    // fix negatives
    if (obj.milliseconds < 0) --obj.seconds, obj.milliseconds = (obj.milliseconds + 1000) % 1000;
    if (obj.seconds < 0) --obj.minutes, obj.seconds = (obj.seconds + 60) % 60;
    if (obj.minutes < 0) --obj.hours, obj.minutes = (obj.minutes + 60) % 60;
    if (obj.hours < 0) --obj.days, obj.hours = (obj.hours + 24) % 24;
    if (obj.days < 0) { // months have different lengths
        --obj.months;
        now.setUTCMonth(now.getUTCMonth() + 1);
        now.setUTCDate(0);
        obj.days = (obj.days + now.getUTCDate()) % now.getUTCDate();
    }
    if (obj.months < 0)  --obj.years, obj.months = (obj.months + 12) % 12;
    return obj;
}

Sounds like you need moment.js

e.g.

moment().subtract('days', 6).calendar();

=> last Sunday at 8:23 PM

moment().startOf('hour').fromNow();

=> 26 minutes ago

Edit:

Pure JS date diff calculation:

var date1 = new Date("7/Nov/2012 20:30:00");
var date2 = new Date("20/Nov/2012 19:15:00");

var diff = date2.getTime() - date1.getTime();

var days = Math.floor(diff / (1000 * 60 * 60 * 24));
diff -=  days * (1000 * 60 * 60 * 24);

var hours = Math.floor(diff / (1000 * 60 * 60));
diff -= hours * (1000 * 60 * 60);

var mins = Math.floor(diff / (1000 * 60));
diff -= mins * (1000 * 60);

var seconds = Math.floor(diff / (1000));
diff -= seconds * (1000);

document.write(days + " days, " + hours + " hours, " + mins + " minutes, " + seconds + " seconds");