タイムゾーンの指定による時刻の変換 From http://d.hatena.ne.jp/tonkoh/20080901/1220287952

convert_zone.rb

class Time
 def convert_zone(to_zone)
  original_zone = ENV["TZ"]
  utc_time = dup.gmtime
  ENV["TZ"] = to_zone
  to_zone_time = utc_time.localtime
  ENV["TZ"] = original_zone
  return to_zone_time
 end
end

output.rb

t = Time.at(1000000000)                   # => Sat Sep 08 21:46:40 EDT 2001

t.convert_zone("US/Pacific")              # => Sat Sep 08 18:46:40 PDT 2001
time.convert_zone("US/Alaska")            # => Sun Sep 08 17:46:40 AKDT 2001
t.convert_zone("UTC")                     # => Sun Sep 09 01:46:40 UTC 2001
t.convert_zone("Turkey")                  # = Sun Sep 09 04:46:40 EEST 2001