Had to revert back to the class_eval approach (for the moment) instead of extending ActiveSupport::Concern.  With the concern approach, DateTime tests (and only DateTime tests) were giving errors like...

    DateTimeExtCalculationsTest#test_to_s:
    ArgumentError: wrong number of arguments (1 for 0)
        /vagrant/rails/activesupport/test/core_ext/date_time_ext_test.rb:14:in `to_s'
        /vagrant/rails/activesupport/test/core_ext/date_time_ext_test.rb:14:in `test_to_s'

Must be something to do with how the module gets loaded for DateTime???  Whatever it is, I can't see it at the moment.  Suggestions welcome.