I've read the description for the DBTIMEZONE function in the official Oracle SQL Language Reference manual. And it's description is straight forward enough. But it's kind of vague. For instance, I executed this query on the Oracle supplied XEPDB1 database that comes with the Express Edition install.
SELECT DBTIMEZONE FROM DUAL;
It comes back as +00:00. Okay, wonderful. What is that timezone offset supposed to reveal to me? That the database was created in London, England? That the database was created in California but given a London, England offset at creation time to make me think it was created in London, England? I'm in the United States, so should I alter the database timezone so it matches my actual timezone? Is it static or dynamic? If static then it could be misunderstood very easily I think.
I have no idea about it's purpose now. The purpose is stated very vaguely in the manual. In a functional way but not a practical way.