A simple, recursive animation algorithm for scrolling an element over a certain duration.

slide.js

function slide(element, value, duration, total) {
    if (!total) { total = 0;}
    if (total >= duration) {return;}
			
    var startLocation = element.scrollLeft;
    var totalDistance = Math.abs(value - startLocation);
    var step = 25;
    var direction = (startLocation < value) ? "left" : "right";
    var iteration = duration / step;
    var iterationDistance = totalDistance / iteration;
			
    if (direction === "left") {
        element.scrollLeft += iterationDistance;
        value += iterationDistance;
    } else {
        element.scrollLeft -= iterationDistance;
        value -= iterationDistance;
    }
			
    setTimeout(function() {slide(element, value, duration, total += step);}, step);
}


/***********************
  usage:
***********************/
var viewport= document.getElementById("my-viewport");
slide(viewport, 500, 200);

Updated after finding flaw in "right" logic. Incremented and Decremented scrollLeft and value values so totalDistance is consistent throughout the animation.