I did find a calendar tool I can use as workaround.
https://fantasy-calendar.com/
https://www.youtube.com/watch?v=MFvY...ature=youtu.be
It does almost all I want. But the tool
- exists outside of RW, so that I still cannot use the date snippets etc.
- needs a work-around to simulate different worlds with a common calendar (i.e. I set up the planet we play on and its moon as additional moons, so that I can at least track the beginn of seasons and the moon phases).
Hopefully, when the Custom Calendar feature finally arrives, it will work somewhat like this, including the settings for sunrise and sunset (on a variety of worlds) and weather.