The United Airlines Website takes your internet time and calculates it for the advance purchase dates. What if I were in a country over the dateline? There is some code built in that it takes from your IP address so for instance 223.343434 could be Paris France and it would take that time into consideration since they have travelers booking from all parts of the globe and besides who wants to be up at 2 AM just to grab a fare that only people in the US can get due to the central time zone where United is based.

Hope this helps and if anyone here can contradict my philosophy go right ahead(: