Adding delay to a scrolling function, more or less allowing it to be less of an instant scroll to a certain position

Question

So, I've got this request and.. Basically I want the scrolling mechanism to be less instant and more of the speed it would be if I naturally scrolled to said position.

Script:

(function() {
    'use strict';
    var SoreHolder = document.getElementById('storeHolder');
    SoreHolder.scrollTop = 0;
    document.addEventListener('keydown', function(e) {
        if (e.keyCode === 90) { // y
            SoreHolder.scrollTop = 1200;
        };
        if (e.keyCode === 67) { // z
            SoreHolder.scrollTop = 2200;
        };
        if (e.keyCode === 72) { // c
            SoreHolder.scrollTop = 1800;
        };
        if (e.keyCode === 84) { // v
            SoreHolder.scrollTop = 1400;
        };
    })
})();

Anyway, if you can offer any help I'd greatly appreciate it. Thank you.