In (certain) out-of-the-box PostgreSQL installations, the timezone GUC is set to "localtime", which seems to mean to query the OS for the value. Unless I'm mistaken, the issue with this is that it doesn't allow clients inspecting the TimeZone GUC to actually know what timezone the server is in, making the GUC largely useless (and creates friction as the GUC can't be expected to always contain valid IANA/Olson values). It would be more useful if PostgreSQL exposed the actual timezone provided by the OS.