user::setup() now stores a DateTimeZone object in user::$timezone representing
the users timezone. For backwards compatibility a numeric value in
user/board_timezone will be converted into one of the legacy Etc/GMT±X
timezones. This will be used until the user updates his/her timezone in the
UCP.
user::format_date() is now basically a legacy wrapper that transforms a UTC
UNIX timestamp into a formatted localised date using phpbb_datetime::format().
PHPBB3-9558