ActiveSupport、Time#last_month
>> require "rubygems" => true >> require "active_support" => true >> Time.local(2008, 1, 1).last_month => Sat Dec 01 00:00:00 +0900 2007
>> require "rubygems" => true >> require "active_support" => true >> Time.local(2008, 1, 1).last_month => Sat Dec 01 00:00:00 +0900 2007