Mongoose is altering unix timestamp and converting it to UTC time

Question

I am following a node, express tutorial. Here is a patch route

app.patch('/todos/:id', (req, res) => {
      var id = req.params.id;
      var body = _.pick(req.body, ['text', 'completed']);

      if (!ObjectID.isValid(id)) {
        return res.status(404).send();
      }

      if (_.isBoolean(body.completed) && body.completed) {
        body.completedAt = new Date().getTime();
      } else {
        body.completed = false;
        body.completedAt = null;
      }

      Todo.findByIdAndUpdate(id, {$set: body}, {new: true}).then((todo) => {
        if (!todo) {
          return res.status(404).send();
        }

        res.send({todo});
      }).catch((e) => {
        res.status(400).send();
      })
});

And here is the test suite for the same route:

it('should update the todo', (done) => {
      var hexId = dummy[0]._id.toHexString();
      var text = 'This should be the new text';

      request(app)
        .patch(`/todos/${hexId}`)
        .send({
          completed: true,
          text
        })
        .expect(200)
        .expect((res) => {
          expect(res.body.todo.text).toBe(text);
          expect(res.body.todo.completed).toBe(true);
          expect(res.body.todo.completedAt).toBeA('number');
        })
        .end(done);
});

Test result:
Error: Expected '2017-07-12T18:38:11.814Z' to be a 'number'

The last test case fails because it is recieving a UTC date from the server instead of good old unix timestamp. But shouldn't it be a timestamp as it is the result of new Date().getTime()?.
I tried logging both body.completedAt() and todo.completedAt() from the route.
body.completedAt() is returning a unix timestamp, whereas todo.completedAt() is returning the UTC time. So it seems that mongoose is altering the timestamp. But how do I prevent that so that the last test case passes.
I am using the expect assertion library

Answer 1

Well, I was stupid enough not to check my mongodb schema: The test case was failing because the type of completedAt was set to Date

completedAt: {
    type: Date,
    default: null
}

It should be:

completedAt: {
    type: Number,
    default: null
}