Testing an ember computed function returns a function not the result of the get

I have a simple Ember object with a computed function which formats a date, e.g.

import Ember from 'ember';
import DS from 'ember-data';

export default DS.Model.extend({
    date: DS.attr('date'),
    posted_date: Ember.computed('date', function() {
        return this.get('date').toLocaleDateString("en-GB");
    })
});

When I test the computed, I weirdly get a failure saying that the the result of get('posted_date') is a function, qUnit outputs it like this:

Expected:   
"2016-05-01"
Result:     
function(){
  [code]
}

Seems like I'm doing something stupid here but I can't figure it out. The value displays ok in my template, so I think there may just be a problem with the test but I can't see anything wrong with it. The test looks like this:

test('date is formatted to UK format', function(assert) {

  const model = this.subject();

  Ember.run(function() {

    model.set('date', new Date('2016-05-01')); 

    assert.equal(model.get('posted_date'), '2016-05-01');
  });
});

Any thoughts?