in rails it gets set in environment.rb via the rails initializer
Rails::Initializer.run do |config|
config.time_zone = 'Pacific Time (US & Canada)'
# ...
I just did a test and when the config.time_zone is commented out Time.zone will also return nil in the rails project; so I guess there is not a 'default' it just gets set in the initializers
Guessing you already know this will 'work'?
irb -r 'rubygems'
ruby-1.8.7-p174 > require 'active_support'
ruby-1.8.7-p174 > require 'active_support/time_with_zone'
ruby-1.8.7-p174 > Time.zone
ruby-1.8.7-p174 > nil
ruby-1.8.7-p174 > Time.zone = 'Pacific Time (US & Canada)'
ruby-1.8.7-p174 > Time.zone
=> #<ActiveSupport::TimeZone:0x1215a10 @utc_offset=-28800, @current_period=nil, @name="Pacific Time (US & Canada)", @tzinfo=#<TZInfo::DataTimezone: America/Los_Angeles>>
Note: above code is using rails 2.2.2 things maybe be different with newer versions?