We launched Office 365 on June 28, 2011. Of course, it was originally codenamed Union and a bundle of our hosted services including Exchange and SharePoint and also included a hosted version of Office 2010.
Plans were launched for small businesses and enterprises, with the enterprise plan adding per-user license for Office 2010. After Office 2013 released, Office 365 updated with it. As time wore on, Office 365 would start including brand new products and go through tons of structural changes, including the introduction of Power BI, the introduction of a Personal plan for single users, and the brand new Planner application.
So, happy birthday, Office 365!
(http://www.zdnet.com/article/microsofts-office-365-turns-five/)