The Book of Enoch has two calendars, one solar, one lunar, and a way to keep them synchronised. I think the Egyptians used this, or a very similar approach around 2000 BC. This is pragmatic because sometimes one calendar is easier and sometimes the other calendar is easier. The sun angle and where it sets and where it rises are needed for the solar calendar but what if the weather is cloudy for a couple of months? Finding the midpoint between extremes of its setting and rising positions is difficult if you do not have much time to devote to it. But it gives you the year length clearly if you can use it. On the other hand the moon phases are easy to keep track of and predictable so you can work them out even if the moon is hidden. But the moon months do not precisely fit into a year. So having the Book of Enoch allows you to use both calendar methods - sun and moon because it tells how to fit lunar months into a solar year. It is slightly inaccurate though and all history has seen the accuracy gradually improved. The intercalary days were simplified in the Book of Enoch. Sophistication and priesthood and astronomy knowledge later in history allowed better intercalary insertions. Today even a second can be added now and then, not just a whole day, because we have computers and atomic clocks. Then there was no such technology but God did not advocate such magic technology and so a simpler version which did not need it was provided in Enoch’s teachings. It worked for a thousand or more years. Egyptians and then later Greeks all knew the solar year was really 365,25 days approximately but technology limitations and human pragmatism prevented them using that knowledge. Only the Romans eventually used it, led by Julius Caesar and Greeks, then Gregory perfected it in the 1500s. 365.242199 days is supposed now to be the tropical solar year length. The Gregory calendar allowed for something close to it. I think the accuracy of Enoch’s calendar fits it well into pre-Egyptian history because later in history the year length was measured more accurately.
