>>213158637>They used lunar months.With the advent of agriculture they had to abandon the lunar calendar, because the climate in northern Europe only allows for one harvest per year and you absolutely have to get the time of sowing right or you'll starve.
While it's easier to count full moons than 365 days plus leap years plus leaving out a leap year ever so often, they had the solar year figured out about 4000 to 3500 years ago. These gold hats have been found in Germany and they are estimated to be between 3800 and 3500 years old. They contain a conversion system between lunar months and solar years denoted by alternating rings and ribbons that are not just ornaments on the hats, but serve a function.
In the neolithicum you could invest decades to move huge stones to form a Stonehenge to observe when the sun rises at the beginning of spring, or you could just wear a golden hat, count the lunar months and use the ornaments of the hat to convert to solar years.