One of the reasons why we create systems of organising and ordering periods of time is that we believe it is important to keep track of dates, events and history.

The way that time is defined can be broken into two categories: natural divisions and ‘constructed' divisions. Natural divisions of time are those ‘created' by nature, like days and seasons. Constructed divisions of time are those created by humans like seconds, minutes and hours or chronologies (see Chronological Table).

In general, people across cultures and time have kept track of time to one degree or another depending on their need for accuracy. For example, a farmer in Mesopotamia may have looked at the sun as it went across the sky during the day to determine the relative time (i.e.: was it closer to sunrise or sunset?). The farmer would not necessarily need to know the exact time during the day in order to perform whatever duties he needed to. However, festivals in Mesopotamia took place on particular days and if somebody borrowed money a particular date was decided for the repayment. A calendar was needed (see Calendar). Mesopotamians also believed it was possible to tell the future but needed a precise understanding of what had happened in the past to work this out (see Astronomy).