kde versions have become a bit too confusing, not quite sure which exact version i have - but it's the latest from suse tumbleweed. when multiple timezones were marked, in earlier kde versions mousewheel on the clock widget would switch them. this does not work anymore - mousewheel does nothing.
It can, see setting on the timezone page in digital clock.
oh ! forgot about that checkbox completely... thank you for the quick response and sorry about the noise :)