Notice that (if I understood correctly) this just sets the timezone for the android compatibility layer and if android applications are using time/date based on system info, they will get correct timezone (as jolla gives GMT timezone as a default when using android applications). I don't know what the other timezone options are exactly or how this affects to play store or any other android software. I happened to fix the Nitrodesk touchdown email/calendar app as it was showing GMT times and I live in EET.

Step by step guide:
enable developer mode and open terminal and enter these commands (see comments for additional info):
devel-su
(privileges...)

pkcon install nano
(install nano editor - http://talk.maemo.org/showthread.php?t=92036)

nano /opt/alien/system.build.prop
(opens up editor and you can add this line at the end of the file:
persist.sys.timezone=Europe/Helsinki
Close editor with ctrl-x and accept to save changes)