Despite the fact that in http://
the user.timezone
property value meets my actual time zone I still have 12-hour clock
Unfortunately, this situation hasn't changed and Jenkins' date display style is still linked to the browser language as of January 2016.
On a sidenote: There's a Locale Plugin, which supposedly allows to override Jenkins' default behaviour of picking up the locale via the browser language. Although - as Greg Dubicki noted - this plugin does process a language_country_variant string, only the language part (a 2-digit ISO 639-1 language code) has an effect, and it's only defining the UI langauge. The date display format still seems to be defined solely by the browser's language.