jQuery UI datepicker binding

http://www.knockmeout.net/2011/07/another-look-at-custom-bindings-for.html

HTML

<link rel="stylesheet" href="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.14/themes/base/jquery-ui.css">
<script src="http://knockoutjs.com/downloads/knockout-2.2.1.debug.js"></script>
<script src="https://raw.github.com/SteveSanderson/knockout-es5/master/dist/knockout-es5.js"></script>
<div data-bind="text: myDate"></div>

<hr />

<button data-bind="click: setToCurrentDate">Set To Current Date</button>

<hr />

<input data-bind="datepicker: myDate, datePropertyName: 'myDate', datepickerOptions: { minDate: new Date() }" />

<hr />

JavaScript

ko.bindingHandlers.datepicker = {
    init: function(element, valueAccessor, allBindingsAccessor, viewModel) {
        var _viewModel = viewModel;
        //initialize datepicker with some optional options
        var options = allBindingsAccessor().datepickerOptions || {};
        $(element).datepicker(options);
          
        //handle the field changing
        ko.utils.registerEventHandler(element, "change", function () {
            var observable = valueAccessor();
            if (!ko.isObservable(observable)) {                
                console.log("Not Observable");
                var datePropertyName = allBindingsAccessor().datePropertyName;
                
                if (datePropertyName) {
                    console.log("Using datePropertyName");
                    observable = ko.getObservable(_viewModel, datePropertyName);
                }
                else {
                    console.log("No datePropertyName, trying _ko_property_writers");
                    var propWriters = allBindingsAccessor()._ko_property_writers;
                    
                    if (propWriters && propWriters.datepicker) {
                        observable = propWriters.datepicker;
                    } else {
                        console.log("No way to get observable");
                        return;
                    }
                }
            }
            observable($(element).datepicker("getDate"));
        });
        
        //handle disposal (if KO removes by the template binding)
        ko.utils.domNodeDisposal.addDisposeCallback(element, function() {
            $(element).datepicker("destroy");
        });
    
    },
    //update the control when the view model changes
    update: function(element, valueAccessor) {
        var value = ko.utils.unwrapObservable(valueAccessor()),
            current = $(element).datepicker("getDate");
        
        if (value - current !== 0) {
           ...