Looks like calling getDate on timepicker does not return an actual Date.
It appears that you can call it using datetimepicker successfully. So, your binding would look like:
ko.bindingHandlers.timepicker = {
init: function (element, valueAccessor, allBindingsAccessor) {
//initialize timepicker with some optional options
var options = allBindingsAccessor().timepickerOptions || {};
$(element).timepicker(options);
//handle the field changing
ko.utils.registerEventHandler(element, "change", function () {
var observable = valueAccessor();
observable($(element).datetimepicker("getDate"));
});
//handle disposal (if KO removes by the template binding)
ko.utils.domNodeDisposal.addDisposeCallback(element, function () {
$(element).timepicker("destroy");
});
},
//update the control when the view model changes
update: function (element, valueAccessor) {
var value = ko.utils.unwrapObservable(valueAccessor()),
current = $(element).datetimepicker("getDate");
if (value - current !== 0) {
$(element).datetimepicker("setDate", value);
}
}
};
Updated sample: http://jsfiddle.net/rniemeyer/L3BNw/