## Bug description

There seems to be an issue with `get_available_timezones_with_offsets`, not sure at what point it broke.

```
Traceback (most recent call last):
    response = view_func(request, *args, **kwargs)
  File "/posthog/posthog/views.py", line 184, in preflight_check
    "available_timezones": get_available_timezones_with_offsets(),
    raise NonExistentTimeError(dt)
```

## Expected behavior
Preflight should work and the timezone list be returned correctly.


## How to reproduce

1. Just check `/_preflight` request

## Environment

- [X] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

## Additional context

Tagging @Twixes as you have more context on this.

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

posthog/utils.py
"""
    Returns an absolutely-formatted URL based on the `SITE_URL` config.
    """
    if not url:
        return settings.SITE_URL
    return urljoin(settings.SITE_URL.rstrip("/") + "/", url.lstrip("/"))
def get_previous_week(at: Optional[datetime.datetime] = None) -> Tuple[datetime.datetime, datetime.datetime]:
    """
    Returns a pair of datetimes, representing the start and end of the week preceding to the passed date's week.
    """
    if not at:
        at = timezone.now()
    period_end: datetime.datetime = datetime.datetime.combine(
        at - datetime.timedelta(timezone.now().weekday() + 1), datetime.time.max, tzinfo=pytz.UTC,
    )  
    period_start: datetime.datetime = datetime.datetime.combine(
        period_end - datetime.timedelta(6), datetime.time.min, tzinfo=pytz.UTC,
    )  
    return (period_start, period_end)
def exception_reporting(exception: BaseException, context: Dict) -> None:
    """
    Determines which exceptions to report to Sentry and sends them.
    """
    if not isinstance(exception, APIException):
        capture_exception(exception)
def relative_date_parse(input: str) -> datetime.datetime: