Here you have to convert back from naive datetime in event timezone, to UTC:

```python
session.start=timezone.localize(start).astimezone(utc).replace(tzinfo=None) if timezone else start
session.end=timezone.localize(end).astimezone(utc).replace(tzinfo=None) if timezone else end
```

It's important to replace tzinfo with None before writing to db, because we use naive datetime columns (PostgreSQL's TIMESTAMP WITHOUT TIME ZONE).