The Calendar: Years
As mentioned earlier, the day is an obvious unit of time for people. But what about weeks, months and years?
Years are fairly straightforward. Man created the concept of a year because seasons repeat on a yearly basis. The ability to predict seasons is essential to life if you are planting crops or trying to prepare for winter. Most plants sprout and bear fruit on a yearly schedule, so it's a natural increment.
A year is defined as the amount of time it takes for the Earth to orbit the sun one time. It takes about 365 days to do that. If you measure the exact amount of time it takes for the Earth to orbit the sun, the number is actually 365.242199 days (according to Encyclopedia Britannica). By adding one extra day to every fourth year, we get an average of 365.25 days per year, which is fairly close to the actual number. This is why we have leap years that are one day longer than normal years.
To get even closer to the actual number, every 100 years is not a leap year, but every 400 years is a leap year. Putting all of these rules together, you can see that a year is a leap year not only if it is divisible by 4 -- it also has to be divisible by 400 if it is a centurial year. So 1700, 1800 and 1900 were not leap years, but 2000 was. That brings the average length of the year to 365.2425 days, which is even closer to the actual number.
The problem with the concept of a year is that it is hard to determine the exact length of a year unless your society has fairly good astronomers. Many cultures that lacked astronomers relied on the cycles of the moon instead. A moon cycle lasts approximately 29.5 days (29.530588 days is the exact number), and it is easy for almost anyone to track the moon's cycle simply by looking at the sky every night.