$ ruby -e 'puts Time.now'
Sat Jan 15 20:49:10 -0800 2011
$ TZ=UTC ruby -e 'puts Time.now'
Sun Jan 16 04:49:20 +0000 2011

def with_time_zone(tz_name)
  prev_tz = ENV['TZ']
  ENV['TZ'] = tz_name
  yield
ensure
  ENV['TZ'] = prev_tz
end

>> with_time_zone('US/Eastern') { puts Time.at(1000000000) }
2001-09-08 21:46:40 -0400
>> with_time_zone('US/Pacific') { puts Time.at(1000000000) }
2001-09-08 18:46:40 -0700

>> require 'active_support/time'
=> true
>> Time.at(1000000000).in_time_zone('US/Eastern')
=> Sat, 08 Sep 2001 21:46:40 EDT -04:00
>> Time.at(1000000000).in_time_zone('US/Pacific')
=> Sat, 08 Sep 2001 18:46:40 PDT -07:00

>> ActiveSupport::TimeZone::MAPPING
=> => {"International Date Line West"=>"Pacific/Midway", "Midway Island"=>"Pacific/Midway", ...}