Only with observation of shadows, then with what are today basic math, geometry and tools you can go a long way. I think stonehenge shows precisely where the sun will set on the day of the solstice? Given that the clock has to show the same time in 1 year from now, you’ll have to divise time in a way to mark each year, each midday, each midnight, why not ad month to it, and prove its accuracy through tests. Basic math but some commitment I guess
Latest Answers