There are about forty different calendar systems currently in use in the world. Some of these systems replicate astronomical cycles according to fixed rules, others are based on abstract, perpetually repeating cycles of no astronomical significance. Some carefully and redundantly enumerate every unit of passing time, others contain mystical ambiguities and metaphysical discontinuities. Some are codified in written laws while others are transmitted by oral tradition.
There are inevitable contradictions and variations in every calendar system—not just because people have remembered wrongly, but because they have remembered differently.
Even if all the calendar systems in the world could be regularized, there would still be difficulties in measuring the passing of time. There are three principal astronomical cycles that secular and scientific calendars attempt to measure. The first is the day which is based on the rotation of the Earth on its axis. Next is the months which is based on the revolution of the Moon around the Earth. Finally, there is the year which is based on the revolution of the Earth around the Sun. The complexity of creating a consistent calendar system arises because none of these cycles of revolution are regular. In other words, they do not comprise an integral number of days. From the vantage of earth, astronomical cycles are neither constant nor perfectly commensurable with each other. The tropical year, for instance, is defined as the mean interval between vernal equinoxes—it therefore necessarily corresponds to the cycle of the seasons. As a result, a calendar year of an integral number of days cannot be perfectly synchronized to the tropical year. Approximate synchronization of calendar months with the lunar phases requires a complex sequence of months of 29 and 30 days. For convenience it is common to speak of a lunar year of twelve synodic months, or 354.36707 days.
The common theme of any and every system is the desire to organize the calendar to satisfy the needs and preoccupations of a particular culture. Besides simply serving the obvious practical purposes, this process of organization provides a sense, however illusory, of understanding and managing time itself. Thus calendars have provided the basis for planning agricultural, hunting, and migration cycles, for divination and prognostication, and for maintaining cycles of religious and civil events. Whatever their scientific sophistication, or lack thereof, calendars are essentially social covenants, not scientific measurements. They are manifestations of a particular worldview or a creed.
And all cultures and the peoples who comprise them, regardless of their scientific, ideological, or creedal protestations to the contrary, necessarily choose a calendar to follow. The only question is what worldview system will they yield to in that choosing. Every calendar system is ultimately a kind of profession of faith.