The reference time zone determines what time your reviews end at. All reviews end at 11:59:59pm in the time zone the account is set in.
*Note: Changing the reference time zone does not affect ongoing reviews. This only applies to future reviews that are launched. If you would like to update reviews that are currently ongoing, you must do an update on the deadline. You are free to use the same exact date, but when you press Save, the new time zone will be registered.