## 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
import datetime
from numbers import Number
from typing import Dict, Optional, Tuple, Union
import posthoganalytics
from celery import shared_task
from dateutil import parser
from dateutil.relativedelta import relativedelta
from django.db import IntegrityError
from posthog.models import Element, Event, Person, SessionRecordingEvent, Team
def _alias(previous_distinct_id: str, distinct_id: str, team_id: int, retry_if_failed: bool = True,) -> None:
    old_person: Optional[Person] = None
    new_person: Optional[Person] = None
    try:
        old_person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=previous_distinct_id
        )
    except Person.DoesNotExist:
        pass
    try:
        new_person = Person.objects.get(
            team_id=team_id, persondistinctid__team_id=team_id, persondistinctid__distinct_id=distinct_id
        )
    except Person.DoesNotExist:
        pass