The syslog-ng Administrator Guide

If the clients run syslog-ng, then use the ISO timestamp, because it includes timezone information. That way you do not need to adjust the recv_time_zone() parameter of syslog-ng.

If you want syslog-ng to output timestamps in Unix (POSIX) time format, use the S_UNIXTIME and R_UNIXTIME macros. You do not need to change any of the timezone related parameters, because the timestamp information of incoming messages is converted to Unix time internally, and Unix time is a timezone-independent time representation. (Actually, Unix time measures the number of seconds elapsed since midnight of Coordinated Universal Time (UTC) January 1, 1970, but does not count leap seconds.)