bower install knockout-partial

// Create date observable
var value = new Date();
var date = ko.observable(value).extend({"partialdate": true});

// Log changes to the console
date.subscribe(function(newValue){ console.log('Date changed', newValue); });
date.year.subscribe(function(newValue){ console.log('Year changed', newValue); });

// Change parts of the date. Each change will update the 'date' observable
date.day(8);
date.month(2);
date.year(1985);

// Change the date. Updates each of the partial observables.
date(new Date());