there you go, using SugarJs for date parsing
var updateInterval = 5 * 1000; // update every 5 seconds
var postDate = new Date(); // this supposed to be the post date
function model() {
var self = this;
this.relativeTime = ko.observable();
setInterval(function() {
self.relativeTime( Date.create( postDate ).relative() );
}, updateInterval);
}
ko.applyBindings( new model() );