ActiveSupport、Time#end_of_month, Time#at_end_of_month
>> require "rubygems" => true >> require "active_support" => true >> Time.local(2008, 12, 1).end_of_month => Wed Dec 31 23:59:59 +0900 2008 >> Time.local(2008, 12, 1).at_end_of_month => Wed Dec 31 23:59:59 +0900 2008