Using newly introduced Joomla! fields (in 3.7 version) the administrator can add some information to each article he or she posts to a website (like start & end date, start & end time, location & description, reminders).
When this information is filled in, a button is shown in the frontend. Using that button you can add the event to your calendar with only few click. From time to time you may be prompted to give permission for the Google api to modify your calendar (if you use the Google calendar) - that's adding a few seconds to the operation, but the general usage is quite simple.
