I’m fairly confidant it is EST (UTC -5), because I have seen it in my correspondence games. It applies to all the clocks, I believe. I think the reason is because anoek, the main dev, and his servers are in this time zone.
I haven’t thought about this, but this means for me the weekend starts 2 PM on Saturday. Not a big deal for me as I play a lot of games without weekend breaks and need to check in anyway (or set vacation). I suspect though if you make the server time more visible, people in Asian/Oceanian timezones might start suggesting some changes.