First, see this topic - http://www.phpbb.com/community/viewtopic.php?f=46&t=2148070

I cannot reproduce that user's exact issue (board default of Pacific (UTC -8) results in Registration default of Mountain (UTC -7)), but on my test installation, if I set the default timezone in the ACP to UTC, the dropdown displays UTC +1 as its default. But if I set it to Pacific Standard time (UTC -8) the dropdown displays Pacific Standard time correctly.