The Calendar As mentioned earlier, the day is an obvious unit of time for people. 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. We all know that it takes about 365 days to do that. 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.