var people = [

  {

    Name: "Bob",

    Birthdate: "1995-12-17T03:24:00Z"

  },

  {

    Name: "Fred",

    Birthdate: "2001-12-17T03:24:00Z"

  }

];

​

var mappedPeople = ko.mapping.fromJS(people, {

    Birthdate: {

        create: function(op) {

            var observable = ko.observable(op.data);

            // Attach computed to 'Birthdate' that produces moment

            observable.moment = ko.computed({

              read: function() {

                return moment( new Date(observable()) );

              },

              write: function(val) {

                observable(val.toDate().toISOString());

              }

            });

            return observable;      

        }

    }

});

​

function VM() {

  this.mapped = mappedPeople;

  // read moment computed

  var moment = this.mapped()[0].Birthdate.moment();

  // manipulate

  moment.add(5, 'y');

  // set it back

  this.mapped()[0].Birthdate.moment(moment);

  // unmapping 

  this.unmapped = ko.mapping.toJSON(this.mapped);

}

​

ko.applyBindings(new VM());