I wanted sunrise and sunset times in Google Calendar so I would know when it's a good time to go out and take a walk. These are all pretty straight-forward calculations that can happen given the lat / long. I created a simple CF workers API that you can paste into "Add other calendars" in the "From URL" action.

You can deploy your own instance using the code at https://github.com/gardner/sunlight

Or, use the reference instance by adding https://cal.sunlight.nz/api/sunrise?lat=-41.27&lon=173.28 (You'll need to use your own lat long)

There is also a calendar that focuses on global events that don't require a lat long. This includes things like lunar phase, equinox, solstice, and cross-quarter days: https://cal.sunlight.nz/api/celestial