>However, it wasn't until the development of sophisticated astronomical observations that the length of a year could be determined accurately, leading to the Gregorian calendar with a year length of 365 days and a leap year of 366 days every four years

people in the Stone age would have had a lunar calendar which was 13 months of 28 days, totaling 364, and they would have realized it was offset every few years and adjusted it accordingly.

This would have been known a LONG, LONG time ago, long before writing, maybe as much as 100,000 years ago

I mean Stonehenge was made around 5000 years ago near the end of the stone age, and that was a masterpiece showing knowledge of leap days and such, and this was before any of the more advanced techniques of the bronze age like water clocks, hour glasses, and more precise sundials that showed up around 3500BC.