Convert time between different time zones

gistfile1.rb

ruby-1.8.7-p352 :002 > #current time zone
ruby-1.8.7-p352 :003 >   Time.zone
 => #<ActiveSupport::TimeZone:0x10e5d8c98 @current_period=nil, @name="UTC", @tzinfo=#<TZInfo::TimezoneProxy: Etc/UTC>, @utc_offset=nil>

irb(main):009:0> d.created_at
=> Fri, 04 May 2012 06:57:18 UTC +00:00

irb(main):012:0> d.created_at.time_zone.name
=> "UTC"

irb(main):013:0> d.created_at.in_time_zone 8
=> Fri, 04 May 2012 14:57:18 CST +08:00

#we can set current time zone

ruby-1.8.7-p352 :005 > u.created_at
 => Tue, 20 Dec 2011 09:20:37 UTC +00:00
ruby-1.8.7-p352 :006 > Time.zone = ActiveSupport::TimeZone[8]
 => #<ActiveSupport::TimeZone:0x10eb0c520 @current_period=#<TZInfo::TimezonePeriod: #<TZInfo::TimezoneTransitionInfo: #<TZInfo::TimeOrDateTime: 684860400>,#<TZInfo::TimezoneOffsetInfo: 28800,0,CST>>,nil>, @name="Beijing", @tzinfo=#<TZInfo::TimezoneProxy: Asia/Shanghai>, @utc_offset=nil>
ruby-1.8.7-p352 :007 > u.created_at
 => Tue, 20 Dec 2011 17:20:37 CST +08:00