## 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/test/test_utils.py
@freeze_time("2020-01-31T12:22:23")
    def test_hour(self):
        self.assertEqual(relative_date_parse("-24h").isoformat(), "2020-01-30T12:00:00+00:00")
        self.assertEqual(relative_date_parse("-48h").isoformat(), "2020-01-29T12:00:00+00:00")
    @freeze_time("2020-01-31")
    def test_day(self):
        self.assertEqual(relative_date_parse("dStart").strftime("%Y-%m-%d"), "2020-01-31")
        self.assertEqual(relative_date_parse("-1d").strftime("%Y-%m-%d"), "2020-01-30")
        self.assertEqual(relative_date_parse("-2d").strftime("%Y-%m-%d"), "2020-01-29")
    @freeze_time("2020-01-31")
    def test_month(self):
        self.assertEqual(relative_date_parse("-1m").strftime("%Y-%m-%d"), "2019-12-31")
        self.assertEqual(relative_date_parse("-2m").strftime("%Y-%m-%d"), "2019-11-30")
        self.assertEqual(relative_date_parse("mStart").strftime("%Y-%m-%d"), "2020-01-01")
        self.assertEqual(relative_date_parse("-1mStart").strftime("%Y-%m-%d"), "2019-12-01")
        self.assertEqual(relative_date_parse("-2mStart").strftime("%Y-%m-%d"), "2019-11-01")
        self.assertEqual(relative_date_parse("-1mEnd").strftime("%Y-%m-%d"), "2019-12-31")
        self.assertEqual(relative_date_parse("-2mEnd").strftime("%Y-%m-%d"), "2019-11-30")
    @freeze_time("2020-01-31")
    def test_year(self):
        self.assertEqual(relative_date_parse("-1y").strftime("%Y-%m-%d"), "2019-01-31")
        self.assertEqual(relative_date_parse("-2y").strftime("%Y-%m-%d"), "2018-01-31")
        self.assertEqual(relative_date_parse("yStart").strftime("%Y-%m-%d"), "2020-01-01")
        self.assertEqual(relative_date_parse("-1yStart").strftime("%Y-%m-%d"), "2019-01-01")
    @freeze_time("2020-01-31")
    def test_normal_date(self):
        self.assertEqual(relative_date_parse("2019-12-31").strftime("%Y-%m-%d"), "2019-12-31")