Given the fact that 3 years passed prior to this change, that the KDE community had to create a fork of libkolab, and that openSUSE dropped the package due to lack of upstream maintenance, there is no point in keeping this task open.

Because jsTimezoneDetect is not very active project I decided to add a workaround on PHP side. Fixed in https://github.com/roundcube/roundcubemail/commit/36485dfc345ad724f08eaa7ea30c3cc88e48a00d.

Copenhagen is, in principle, available. The reason for the parent ticket to exist has largely been obsoleted. This sub-ticket, and its sub-tickets, can therefore be closed.

I have filed now a bug at the jstz library, because I guess that would be the cleanest solution.
If that ignores Etc timezones coming from the internationalization API from the browser, then it calculates a proper Olson timezone.

It does not depend on Windows Server version.
It does not depend on Firefox version either.

I got more details: the client is Firefox 56 running on Windows Server 2012 R2. The timezone setting for the user in Roundcube is "auto".

Done.

https://git.kolab.org/rLKXab256ebd90d3cf7186c44211a40e2602019c67c9

The attach property is indeed optional, but libkolabxml doesn't treat it as optional property.

https://git.kolab.org/rLK65aaf8504a5fd0bc6775ef52686bf1c8a9e3b5bb

This will need to be packaged and tested in a setup where this actually happens.