dayjs object with timezone GMT 0 return (maybe?) wrong date when calling .startOf('date')

**Describe the bug**
Adding a timezone GMT 0 to a date and trying to get the start of the day return date with the wrong hour.
Maybe I don't fully understand timezones, but from was I understand, the function `startOf` should give me the start of the current day - which is 00:00.

Why would GMT 0 behave any different? does it have any special property I am not aware of?

on any case, here is a sandbox example with different dayjs object with different timezones, and all return the same values when calling `startOf` , except GMT 0

**Expected behavior**
getting 00:00 when calling `startOf('date')`

**Information**
- Day.js Version 1.11.5
- OS: MacOS
- Browser chrome
- Time zone: GMT +02:00