## Bug description
 
It looks like either timestamp or sent_at is being sent without timezone, causing [this issue](https://sentry.io/organizations/posthog/issues/1972106098/?project=1899813&query=is%3Aunresolved&statsPeriod=14d)

## Expected behavior

It should send both timestamps with timezone

## How to reproduce

1. 
2. 
3. 

## Environment

- PostHog cloud or self-managed?
- PostHog version/commit

## Additional context



#### *Thank you* for your bug report – we love squashing them!

posthog/tasks/process_event.py
try:
        person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=str(distinct_id)
        )
    except Person.DoesNotExist:
        try:
            person = Person.objects.create(team_id=team_id, distinct_ids=[str(distinct_id)])
        
        except:
            person = Person.objects.get(
                team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=str(distinct_id)
            )
    person.properties.update(properties)
    person.save()
def _set_is_identified(team_id: int, distinct_id: str, is_identified: bool = True) -> None: