Date filter with google-cloud-datastore misses a lot of objects

I have a standard Python App Engine service, and a flexible service (in the same project) written in node.js. The flexible service is an admin dashboard.

I'm trying to filter on a datetime property field, and I know I can do it on the Python side and get valid results, but it fails in the node.js service.

Python:

endTime = datetime.today().replace(hour=0, minute=0, second=0, microsecond=0)
startTime = endTime - timedelta(1)

newusers = SVCUser.query(SVCUser.created_at >= startTime,
                         SVCUser.created_at < endTime).count(keys_only=True)

Node.js:

const startTime = moment().utc().subtract(1, 'days').startOf('day').toDate();
const endTime = moment().utc().startOf('day').toDate();

const query = ds.createQuery('SVCUser')
      .filter('created_at', '>=', startTime)
      .filter('created_at', '<', endTime);

return ds.runQuery(query)
    .catch((err) => {
        return logging.error(err);
    })
    .then((results) => {
        const data = {
            event_date: endTime,
            new_users: results[0].length > 0 ? results[0].length : 0
        };

        return bigquery.dataset('stats').table('newusers').insert(data);
    });

The result of my query is always nonsense: either zero entries, or if I move the date range around a bit, one or two entries. I've verified with the GAE console that I should have 100+ a day, which is also what the Python code returns.

The field in question is indexed, so I don't know what's going on here.

class SVCUser(ndb.Model):
    [...]
    created_at = ndb.DateTimeProperty(auto_now_add=True)

via Faried Nawaz