helper

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  model: {
    date: new Date().toString(),
  	oneSecondLate: new Date().toString()
  },
  init() {
  	this._super(...arguments);
    
    //issue
    setInterval(() => {
      	Ember.set(this, 'model.date', new Date().toString());
      new Ember.RSVP.Promise(resolve => {
        let date = new Date().toString();
      	setTimeout(() => {
        	resolve(date)
        }, 1000);
      }).then(data => {
      	Ember.set(this, 'model.oneSecondLate', data);
      //solution
      this.notifyPropertyChange('model');
      });
    }, 1000);
  }
});

helpers.my-helper.js

import Ember from 'ember';

export function myHelper(params/*, hash*/) {
  let obj = Ember.get(params, `firstObject`);
  let prop = Ember.get(params, '1');
  return Ember.get(obj, prop);
}

export default Ember.Helper.helper(myHelper);

templates.application.hbs

model.date: {{model.date}} <br/><br/>
model.testProp: {{model.oneSecondLate}} <br/><br/>
helper: {{my-helper model 'oneSecondLate'}}, <br/>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}