The develop of a calendar is something quite recent in Eden.
The modern calendar introduced in the 4888 is called "The Artificial Calendar" because is based on the calculation of a day using the Standard Second (with a time value close to the Earth's second). It quickly become a standard for many nations (because it was fixed, and not subject to astronomical variations).
The Calendar is divided in:
Days: each day is 30 hours long.
Cycle: each cycle is composed by 30 days (the meaning can br compared to Earth's Month)
Year or Revolution: composed by 18 cycles