Bug 1054818 - [Sora][Call][Settings]The call time display abnormal after...

... user change the system time.
I don't like how we went from not stubbing ```Date.now()``` to stubbing it in the middle of the test. I think we should use this pattern instead:
```js
this.sinon.stub(Date, 'now').returns(12000);
// ...
Date.now.returns(6000);
// ...
```js
That way, we're not relying on any real timing.