Valve often seems to release stuff at a time that's convenient to them; as they're on the other side of the US, that can often translate into late evening or tomorrow for the rest of the world, which is why I'm guessing they picked a Thursday to release it rather than the more usual Friday. Which is pretty much what I'm aiming for, especially if they don't activate pre-loading as my internet is not very quick...
Actually developers determine the release date and time. Valve gives the option. If they just choose a date it is released at 12AM EST on the chosen date. So if Larian decided to release it on 9/14 with no given time, it will be released at 9/14 12AM EST. Which is good news for me because then I would get it 9/13 9PM PST
The only time this is not true is if there is clause in a distribution contract that says Valve can't release the game until retail stores are open.
[edit] If they release it 9/14 Midnight CEST that would be 9/13 6PM EST or 9/13 3PM PST
