I don't think they have servers on the East Coast. You choosed your server when you installed your TWS (location closest to you) You can switch off the server time. (You still get the time in your executio report)
I think the difference is in the IP connection? When I connect from a different IP, same account, I end up on CST, while at home I am on EST. The execution reports I get are also given in CST, which can be confusing at times.