Actually you can change that without access to the config file,
timezone is read from your login information
when you login the roundcube webmail, the following data is post(using burpsuite you can see)
"_token=wMSUGT3I5nY20J4rTGDOIXsGL75wKRIi&_task=login&_action=login&_timezone=Asia%2FShanghai&_url=&_user=****&_pass=****"
you just need to change "_timezone", and timezone is defined in file
/roundcubemail-1.3.6/program/lib/Roundcube/rcube_config.php