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.)
Starting with version 3.2, the syslog-ng Open Source Edition application is licensed under a combined LGPL+GPL license. The core of syslog-ng OSE is licensed under the GNU Lesser General Public License Version 2.1 license, while the rest of the codebase is licensed under the GNU General Public License Version 2 license.
NOTE: Practically, the code stored under the lib directory of the source code package is under LGPL, the rest is GPL.
For details about the LGPL and GPL licenses, see GNU Lesser General Public License and GNU General Public License, respectively.
Multiple syslog-ng servers can be run in fail-over mode. The syslog-ng application does not include any internal support for this, as clustering support must be implemented on the operating system level. A tool that can be used to create UNIX clusters is Heartbeat (for details, see this page).
The following sections describe the structure of log messages. Currently there are two standard syslog message formats: