The mentioned solution can be found in this thread:
https://forum.test.turris.cz/t/time-difference-in-system-log/1175/22
Unfortunately this fix is applied in /etc/syslog-ng.conf, so it gets overwritten from time to time. Could some provide the correct syntax for a config to put in a separate file under /etc/syslog-ng.d/ ?
Thank you!