Hello,
I noticed that when changing the default locale (e.g., from English to French), the dates are affected. I do not think they should, given that languages and timezones are not supposed to be linked – you can speak a language regardless of your location, and most colonizer languages are spoken across timezones.
E.g.,
given the same date entry on various instances using multisite (or not – but it also fails on the same multisite install), from the same computer with the same timezone configured for the user:
[date=2020-12-04 time=10:00:00 timezone="Europe/Brussels"] → [date=2020-12-04 time=15:00:00 timezone="Europe/Brussels"]
On English by default sites I get the expected time :
While on French by default instances the time offset is wrong:
Here’s what shows up here in Meta:
2020-12-04T09:00:00Z → 2020-12-04T14:00:00Z
I noticed the offset on earlier version but only made the link with default locale setting now, so I cannot report an affected version. I just tried on the stable: it’s affected with French but not with Spanish, so maybe it’s only a French thing – which would not be surprising, the French have this certainty of controlling spacetime. 
cc @zogstrip