ActiveSupport、Date#advance
>> require "rubygems" => true >> require "active_support" => true >> Date.new(2008, 12, 31).advance(:years => 1) => Thu, 31 Dec 2009 >> Date.new(2008, 2, 29).advance(:years => 1) => Sat, 28 Feb 2009 >> Date.new(2008, 12, 31).advance(:years => 1, :days => 1) => Fri, 01 Jan 2010