Yet another improvement for date comparing

It's me again. I did tested it on different cases and found a small problem:

![window](https://cloud.githubusercontent.com/assets/52201/4541509/0f4c270c-4e15-11e4-8eff-fee1f805dd09.png)
This is because in this case I've used `Date` mocked with Sinon.JS so constructor is not equal to current value of `Date`.

Best way to avoid problem with successors it's to use `instanceof` instead of `constructor`.

I tried this patch with my code and it works just fine. I think we good here.