This handles parsing of a duration string in javascript which allows for simple arithmetic.

duration.js

/* Duration is a class that allows adding to or subtracting from dates.
 * It's backed by a javascript date object, so this only works for milliseconds,
 * not, microseconds.
 * 
 * Note: This only deals with hours, minutes and seconds up to 23:59:59.999
 * 
 * * param duration: the string duration value. (i.e. "01:25:35.123")
 * 
 * Examples:
 * 
 * >>> new Duration('12:35:45.123').getTimeString()
 * "12:35:45.123"
 * >>> new Duration('12:35:45.123').addSeconds(5).getTimeString()
 * "12:35:50.123"
 * >>> new Duration('12:35:45.123').addMinutes(2).addSeconds(5).getTimeString()
 * "12:37:50.123"
 * >>> new Duration('12:35:45.123').addMinutes(2).addSeconds(5).addMilliseconds(100).getTimeString()
 * "12:37:50.223"
 * >>> new Duration('5:45.123').addMinutes(2).addSeconds(5).addMilliseconds(100).getTimeString()
 * "0:07:50.223"
 * >>> new Duration('5:45.123').addMinutes(2).addSeconds(5).getTimeString()
 * "0:07:50.123"
 */
function Duration(duration){
    var hours = 0,
        minutes = 0,
        seconds = 0,
        milliseconds = 0;
    
    if (duration != undefined) {
        parsed = duration.split(':');
        parsed.reverse();
        
        hours = (parsed.length > 2) ? parseInt(parsed[2]) : 0;
        minutes = (parsed.length > 1) ? parseInt(parsed[1]) : 0;
        seconds = parseInt(parsed[0]);
        milliseconds = this.parseMilliseconds(parsed[0]);
        
    }
    this.date = new Date(0, 0, 0, hours, minutes, seconds, milliseconds);
}

// Get the milliseconds from a time string.
Duration.prototype.parseMilliseconds = function(seconds) {
    if (seconds.indexOf('.') === -1) {
        // no decimal found, no milliseconds
        return 0;
    }
    
    milli_string = seconds.split('.')[1];
    
    if (milli_string.length > 3) {
        milli_string = milli_string.slice(0, 3);
    } else if (milli_string.length < 3) {
        while (milli_string.length < 3) {
            milli_string = milli_string + '0';
        }
    }
    
    return parseInt(milli_string);
}

/* Get formatted string time.
 *
 * 1:25:35.123
 */
Duration.prototype.getTimeString = function() {
    
    var hours = this.getHours(),
        minutes = this.getMinutes(),
        seconds = this.getSeconds(),
        milliseconds = this.getMilliseconds(),
        output_time = '';

    output_time += hours + ':' +
    			   this.leftPad(minutes, 2) + ':' + 
    			   this.leftPad(seconds, 2);
    
    if (milliseconds > 0) {
        output_time += '.' + this.leftPad(milliseconds, 3);
    }
    
    return output_time;
}

Duration.prototype.leftPad = function(pad_this, num_times, pad_with) {
	var output = pad_this.toString(),
        pad_with = (!pad_with) ? '0' : pad_with;
    
	while (output.length < num_times) {
		output = pad_with + output;
	}
    
	return output;
}

Duration.prototype.getHours = function() {
    return this.date.getHours();
}

Duration.prototype.setHours = function(hours) {
    this.date.setHours(hours);
}

Duration.prototype.addHours = function(hours) {
    this.date.setHours(this.date.getHours() + hours);
    return this;
}

Duration.prototype.getMinutes = function() {
    return this.date.getMinutes();
}

Duration.prototype.setMinutes = function(minutes) {
    this.date.setMinutes(minutes);
}

Duration.prototype.addMinutes = function(minutes) {
    this.date.setMinutes(this.date.getMinutes() + minutes);
    return this;
}

Duration.prototype.getSeconds = function() {
    return this.date.getSeconds();
}

Duration.prototype.setSeconds = function(seconds) {
    this.date.setSeconds(seconds);
}

Duration.prototype.addSeconds = function(seconds) {
    this.date.setSeconds(this.date.getSeconds() + seconds);
    return this;
}

Duration.prototype.getMilliseconds = function() {
    return this.date.getMilliseconds();
}

Duration.prototype.setMilliseconds = function(milliseconds) {
    this.date.setMilliseconds(milliseconds);
}

Duration.prototype.addMilliseconds = function(milliseconds) {
    this.date.setMilliseconds(this.date.getMilliseconds() + milliseconds);
    return this;
}