A new time zone field has been added to the Account settings page. When you change your time zone, all future reports will be based on the new setting. It’s important to note, that the change is not retroactive. Your reports prior to the change will still be based on Pacific Standard Time.
You’ll also want to think about the time of day that you make the switch, as it will affect your reporting for that day.
Google announced that the change will roll-out of the next couple of weeks.