Hi, I want to know how many days a month has. So I lookup here: http://www.ruby-doc.org/core/classes/Time.html CommonYearMonthDays and LeapYearMonthDays constants do what I need. But I cannot access them. $ irb irb(main):001:0> require "time" => true irb(main):002:0> print Time::CommonYearMonthDays NameError: uninitialized constant Time::CommonYearMonthDays from (irb):2 But I can access constants from another module, such as BLOCK_SIZE from openssl/buffering module. Any idea? Ruby 1.8.4 ( build from source ). Thank you.