## 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
try:
        return datetime.datetime.strptime(input, "%Y-%m-%d").replace(tzinfo=pytz.UTC)
    except ValueError:
        pass
    
    
    try:
        return parser.isoparse(input).replace(tzinfo=pytz.UTC)
    except ValueError:
        pass
    regex = r"\-?(?P<number>[0-9]+)?(?P<type>[a-z])(?P<position>Start|End)?"
    match = re.search(regex, input)
    date = timezone.now()
    if not match:
        return date
    if match.group("type") == "h":
        date = date - relativedelta(hours=int(match.group("number")))
        return date.replace(minute=0, second=0, microsecond=0)
    elif match.group("type") == "d":
        if match.group("number"):
            date = date - relativedelta(days=int(match.group("number")))
    elif match.group("type") == "m":
        if match.group("number"):
            date = date - relativedelta(months=int(match.group("number")))
        if match.group("position") == "Start":