Google Calendar is a time-management and scheduling calendar service developed by Google. It was created by Mike Samuel as part of his 20% project at Google.[5][6] It became available in beta release April 13, 2006, and in general release in July 2009, on the web and as mobile apps for the Android and iOS platforms.

Google Calendar allows users to create and edit events. Reminders  can be enabled for events, with options available for type and time.  Event locations can also be added, and other users can be invited to  events. Users can enable or disable the visibility of special calendars,  including Birthdays, where the app retrieves dates of births from  Google contacts and displays birthday cards on a yearly basis, and  Holidays, a country-specific calendar that displays dates of special  occasions.

Over time, Google has added functionality that makes use of machine learning, including "Events from Gmail", where event information from a user's Gmail messages are automatically added to Google Calendar; "Reminders", where  users add to-do activities that can be automatically updated with new  information; "Smart Suggestions", where the app recommends titles,  contacts, and locations when creating events; and "Goals", where users  enter information on a specified personal goal, and the app  automatically schedules the activity at optimal times.

(From Wikipedia, the free encyclopedia)