new Date(epoch) returning invalid date inside Ember component

Question

I have a date-filter component that I am using in my Ember application that only works on initial render, not on a page reload, or even if I save a file (which triggers the application to live update).

In the main template of my application, I render the date-filter like this passing it a unix timestamp

{{date-filter unixepoch=item.date}}

Then, in components/date-filter.js, I use a computed property called timeConverter to change the unix epoch into a time string formatted according to user's language of choice, and then in my templates/components/date-filter.hbs file I do {{timeConverter}} to display the results

timeConverter: function(){
    //step 1: get the epoch I passed in to the component
    var epoch = this.get('unixepoch');
    //step 2: create a human readable date string such as `Jun 29, 2015, 12:36PM`
    var datestring = new Date(epoch)

    //do language formatting --code omitted as the problem is with step2

}

It is step 2 that fails (returning invalid date) if I refresh the page or even save the file. It always returns the proper date string the first time this component is called. Even if I do new Date(epoch) in the parent component, and try to pass the result in to this component (to do foreign language formatting), I'm having the same problem.

Question: how can I figure out what's happening inside new Date(epoch), or whether it's an issue related to the component?