I am using KnockoutJS to bind some values. I have a date value TimeByDay that I bind like this:

<span data-bind="text: new Date(TimeByDay)"></span>

But I would also like to transform the date on a different location and simply add a day to the TimeByDay value.

I can do it like this:

<span data-bind="text: new Date(new Date(TimeByDay).setDate(new Date(TimeByDay).getDate() + 1))"></span>

Is there a prettier way to do this? Maybe some feature in KnockoutJS that I am missing?

(The reason I am encapsulating both values in a new Date() construct is that I also apply formatting to the date.)


$(document).ready(function () {

    var serverData = $.getJSON("../../_api/");
    serverData.done(function (data) {
        ko.applyBindings(new EngagementsList(data.value));


function EngagementsList(engagementList) {
    var self = this;
    self.entries = engagementList;


<span data-bind="foreach: entries">
  <span data-bind="text: new Date(TimeByDay)"></span>
  <span data-bind="text: new Date(new Date(TimeByDay).setDate(new Date(TimeByDay).getDate() + 1))"></span>

1 Answer 1


One way would be to move the date display logic to your view model.

function vm(){
    var self= this;

    self.TimeByDay = ko.observable(new Date("December 17, 1995 03:24:00"));

    // fn extention of ko.observable        
    ko.observable.fn.AddOneDay = function(){
        return new Date(this().setDate(this().getDate()+1))

    self.AddOneDay = function(TimeByDay){
        return new Date(TimeByDay().setDate(TimeByDay().getDate()+1))

And then in your view you can have

<span data-bind="text: TimeByDay"></span>
<span data-bind="text: AddOneDay(TimeByDay)"></span>


<span data-bind="text: TimeByDay.AddOneDay()"></span>

Use the extention fn method if your application requires 1 day added to date frequently. Read this for more.

Jsfiddle: https://jsfiddle.net/newuserjs/oLh6u67z/4/

  • \$\begingroup\$ Hi @Dandy, the .fn extension point was exactly what I was looking for. So I have added that, but cannot get it to work referencing a property of the items being iterated over. My workaround is to declare a global function (and thereby bypassing konckoutjs), but I'd much rather use the extension point, but I cannot get past the exception "'AddOneDay' is undefined". jsfiddle.net/oLh6u67z/17 . Any thoughts? \$\endgroup\$ Commented Sep 7, 2015 at 9:50
  • \$\begingroup\$ @JesperLundStocholm you get 'AddOneDay' is undefined because the 'fn' extention point is only for ko.observables. It wont work for strings or other types. You can convert your data to observable array using knockout-mapping plugin, and then use the 'fn' extention method to your TimeByDay observable. jsfiddle.net/newuserjs/oLh6u67z/19 \$\endgroup\$
    – Dandy
    Commented Sep 7, 2015 at 11:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.