"Clive D.W. Feather" <clive@demon.net> writes:
However, the upper horizontal line on that diagram roughly follows the Great Caucasus Mountains, which makes a reasonable boundary.
The key question here is what continent Tbilisi is in, since that's the data point that the tz database uses. Modern sources typically give either the crest of the Caucasus, or the Kuma-Manych Depression (which separates the Russian Plain from the North Caucasus), as the boundary between Europe and Asia in that region. This would put Tbilisi in Asia. For ancient sources the story is more interesting. Prof. Thomas V. Gamkrelidze, Member of the Georgian Parliament, writes in <http://www.parliament.ge/GENERAL/C_D/europe.htm> that for the ancient Greeks the boundary was the river Phasis, normally identified with the modern Rioni. Gamkrelidze must be alluding to Herodotus, Histories, IV.45 <http://www.perseus.tufts.edu/cgi-bin/ptext?lookup=Hdt.+4.45.1> but he omits some crucial details. Herodotus wrote that the border between Europe and Asia is not universally agreed upon, and that some say it's the Don, some the Kerch straits, and some (including Herodotus himself) the Phasis river. All these interpretations would mean that Tbilisi itself in Asia, even though Herodotus's preferred border (the Phasis/Rioni) would mean that some of Georgia is in Europe. But wait, there's more! Gamkrelidze writes that some people identify the Phasis with the modern Chorokhi River. This extreme interpretation would put most of Georgia (including Tbilisi) in Europe (along with some of northeast Turkey, and perhaps Armenia and a few other places....). However, I haven't been able to find any other source that makes this claim: all the standard sources I have found identify the Phasis with the modern Rioni. This shouldn't be too surprising, since the Rioni flows into the Black Sea at Poti, and Poti's ancient name was also Phasis. I'm pretty dubious about any attempt to identify the Phasis with the Chorokhi, and if you read Gamkrelidze's article you'll see he's pretty dubious about that too. While we're on the subject of ancient sources, Strabo, Geography, XI.1 <http://www.perseus.tufts.edu/cgi-bin/ptext?lookup=Strab.+11.1.1> says flatly that the boundary between Europe and Asia is the Don. To summarize the last few paragraphs: all the ancient sources I've found agree that Tbilisi itself is in Asia.