Ask HN: What are the best open source calendar APIs?

I am specifically interested in open source calendar APIs that connect to the popular online calendars: Google, Outlook, Office 365 or iCloud. It would be a an extra special benefit if the API also integrated with a free software or open source calendar. The API must offer the ability to query a user's availability as well as create new events / appointments.

I am aware of cronofy, but the pricing is a bit steep for my purposes: https://www.cronofy.com/pricing/