Hubspot date fix

from everything I've read in the dev group, it _looks_ like HubSpot accepts only millisecond times for the REST APIs. right now, we pass everything straight to the javascript on the frontend, but we probably should be transforming dates into millisecond timestamps

@jessbrandi does that sound right? i'm not quite sure from the [javascript api docs](https://developers.hubspot.com/docs/methods/enterprise_events/javascript_api). thanks!