How to slow down and speed up time in JavaScript

Question

I implemented an example of how to pause time in JavaScript. The example is here http://jsfiddle.net/suska/n4g5U/

// Update of Date class to modify getTime method.
Date.prototype.origGetTime = Date.prototype.getTime;
Date._lastPausedAt;
Date._stopDuration = 0;
Date.prototype.getTime = function() {
    if (Date._lastPausedAt) {
        return Date._lastPausedAt.origGetTime() - Date._stopDuration;
    }
    return new Date().origGetTime() - Date._stopDuration;
};
Date.isPaused = function() {
    return Date._lastPausedAt != null;
};
Date.pause = function() {
    if (!Date._lastPausedAt) {
        Date._lastPausedAt = new Date();
    }               
};
Date.unpause = function() {
    if (Date._lastPausedAt) {
        Date._stopDuration += new Date().origGetTime() - Date._lastPausedAt.origGetTime();
        Date._lastPausedAt = null;
    }
};

Any idea how can I modify the example to do a slow down and speed up functionality?

Answer 1

Wrote a variant on dooxe's answer to avoid the sudden jumps when changing between time warps.

var milli = Date.prototype.getTime;
var lastTime = (new Date).getTime();
var curTime = 0;
Date.prototype.getTime = function(){
    var actualTime = milli.call(this);
    curTime += (actualTime - lastTime) * Date._speed;
    lastTime = actualTime;
    return curTime;
};

Answer 2

Here is a trick to 'override' Date.getTime function :

var milli = Date.prototype.getTime;
Date.prototype.getTime = function(){
    return milli.call(this) * 5;  
};

Use a factor (5 in my code) inferior to 1 to slow the time :

http://jsfiddle.net/uKk9R/