Made in 2020.3.3f1.
EDIT: I've tried this in more recent versions of Unity and it is completely broken. I would recommend not bothering with HDRP and use the SRP.
Gives a physical sky with rising and setting sun, moving directional light, starfield, and phases of moon with varying luminosity.
Also includes a basic time keeping system to allow the definition of day length and a speed up factor for night time. I'll try and add a YouTube demo when I have time.