Why is Ruby’s Date class automatically loaded but DateTime is not?

In IRB, include this line: require 'date' then you will be able to use DateTime.

irb(main):000:0> DateTime.class
NameError: uninitialized constant DateTime
        from (irb):0
        from /path/to/ruby/irb:12:in '(main)'
irb(main):001:0> require 'date'
=> true
irb(main):002:0> DateTime.class
=> Class