Today I got an alert stating that my firewall was not logging anymore, upon investigation it appears only my Fortigate logs are having their time stamps changed by 4 hours. This started happening after I applied the last update, though I made no changes to either system and previously this has been working for the last year properly.
As you can see in the attached picture I have the correct time stamp in the log, but graylog is not parsing that time correctly it appears. I have verified system time, graylog user time, and firewall time and they all match.
FWIW, Graylog now tries to use the timestamp provided in the Syslog messages from FortiOS instead of simply using the ingest time, but it assumes the timezone to be UTC.
Please provide some raw Syslog messages from your FortiGate devices (captured via Wireshark or TCPdump) so we can test them against the current syslog parser.