So I coded a tiny little 512-byte program fitting into a bootsector, which displays a big clock.
I even managed to squeeze switchable fore- and background colors and a key to toggle 12 vs 24 hour display in there. Now the sector is filled up to the limits, only 1 byte left. That’s 509 Bytes of Code & Data for a functioning clock program + 2 Bytes for the bootsector marker.
I pushed the NASM source code over to my new created github. If you want to try it, follow the instructions there. There is many more to optimize, I encourage anyone to go and try it :)