When you're trying to tune your sundial and it starts down pouring between the last 2 pegs...
My guesses below are way off, I took a screenshot at noon, timing 2m 15s and marking true hours in a photo editor where the markers SHOULD be. I'll probably never use it for its actual purpose, so I'm thinking of re-purposing the floor for a different building concept.
If you placed the gnomon due north then north is 12pm that's all I needed for mine lol. The closer the shadow got to 90 degrees from 12 I knew it was getting late in the day. I actually used my Sundial regularly while doing my survival playthrough