It seems that you are using full database now.
The 10y database doesn't contain time zone abbreviations.
And user.timezone can be CET or other abbreviations.
If you are going to use the 10y database for better performance, you might need some kind of mapping.

See https://github.com/zulip/zulip/blob/42bc59b8027a074b31054f5fc0f1e6440fdd5252/zerver/lib/timezone.py#L22-L24

Also from https://pub.dev/packages/timezone#location.

We don't provide any functions to get locations by time zone abbreviations because of the ambiguities.

@tannal I see, thank you!