Both Merriam-Webster and Oxford says it's acceptable: http://www.merriam-webster.com/dictionary/millenniums http://www.merriam-webster.com/dictionary/millenniu

Both Merriam-Webster and Oxford says it's acceptable: http://www.merriam-webster.com/dictionary/millenniums http://www.merriam-webster.com/dictionary/millenniums http://www.oxforddictionaries.com/definition/english/millennium http://www.oxforddictionaries.com/definition/english/millenn...