GMT offset used in local time formats was computed at startup, but was not updated when DST status changed while running.
For example these two RFC5424 syslog traces where emitted 5 seconds apart, just before and after DST changed:
<14>1 2016-03-27T01:59:58+01:00 bunch-VirtualBox haproxy 2098 - - Connect ...
<14>1 2016-03-27T03:00:03+01:00 bunch-VirtualBox haproxy 2098 - - Connect ...
It looked like they were emitted more than 1 hour apart, unlike with the fix:
<14>1 2016-03-27T01:59:58+01:00 bunch-VirtualBox haproxy 3381 - - Connect ...
<14>1 2016-03-27T03:00:03+02:00 bunch-VirtualBox haproxy 3381 - - Connect ...
This patch should be backported to 1.6 and partially to 1.5 (no fix needed in log.c).