Given the various racial histories of Etherias, the oldest (and least understood) is the Draconic Calendar (referred to as Primordial Epoch - PE).
The Draconic Calendar
Like almost all which follow it, is based on the time for the planet to revolve around the sun (365 days) and Seasons. There appear to be no discernible days, weeks or months. The earliest known reference date according to Helani sages would appear to be around 52,000 years ago and references the "Breaking of the Shell". Almost all references to the Draconic Calendar are found on ancient stone tablets, many of which are massive in size.
Sages of all races treat this as the beginning of the Primordial Epoch, when the reptilian races dominated this world. However the noted Dwarven sage Thororn Burast of Stoneheart claims to have found evidence of earlier epochs.
The Atheviri Calendar
Around 24,000 years ago, traces of Atheviri writings can be found inscribed on crystalline tablets. These ultra hard tablets are commonly regarded as unbreakable, but can be dissolved with strong acids. Rarely libraries full of these tablets are discovered in Athevirian ruins, they universally have a "date" in the top right hand corner in the circular Atheviri alphabet. The date includes days (one rotation of the planet) and twelve months (30 days each), with 5 "special" days added at different points of the year, depending on which year.
The earliest "dates" on these tablets are commonly held to be the start of "The Dawning" epoch, when the Atheviri arose to become the dominant race. Currently no crystal tablets with a date after 12,500 DE (Dawning Epoch, 40000 PE) have been found, so this is regarded as the likely end of the Dawning.
The Helani Calendar
With the rise of the Helani (High Elven) 6200 years we ago, a new calendar was created upon the orders of Arillian the first God-King (later to become the dynasty of God-Emperors in an unbroken line). The calendar dates from his birth 6200 years ago and introduces the concept of seven day long weeks to accommodate worship days (when the worship of the Godking was mandatory). The number seven was chosen as it was regarded as a "holy" number by Arillian, for his people one day in seven was given to the God-King as was his right. With the eventual ascendancy of Arillian to his place in the heavenly realms, the Helani calendar was restarted under his daughter Cellanel. Therefore dates are expressed as a number followed by the emperors name, e.g. 210 Arillian, 52 Cellanel and so on.
For more serious scholarly historical work in the Eternal Empire, the birth of Arillian is the origin date with everything before it being either Before Arillian (BA) or After Arillian (AA). The birth of Arillian was in 45,800 PE (17,800 DE) and by many sages is held to represent the start of the "Age of Ascendancy", when Elves, Dwarves and Giants rose to rule the world.
The Calendars of Giantkind
Little is known or recorded about the calendars kept by the more advanced giant races. Given the long lived nature of those races and the perceived wisdom of the Cloud Giants in particular, some form of timekeeping must exist.
The Stoneheart Calendar
To mark the founding of Stoneheart, the first city and spiritual home of the Dwarven peoples, the first "Stone of Ages" was laid. These cube shaped numbered stones are laid once a century (100 years), to mark the passing of time for the Dwarves, in an ever increasing spiral at the centre of Stoneheart. The Dwarven mathematician and scholar Ulam Hardheart noted they form a "prime spiral", which he believed would reveal the secret numbers to decipher the as yet unreadable high language of the Atheviri.
Stoneheart was founded 5700 years ago according to the Stones of Ages, so the current date is 5700 SA (Stone Age, 56300 PE, 18300 DE, 500 AA). With such hard to dispute, well maintained physical evidence, the Stones of Ages are highly regarded as a dating system by all but the Helani scholars. Given their underground nature, seasons are not named or documented by the Dwarven scholars, however they were the first race to establish 60 minutes in an hour and 60 seconds in a minute. Prior to this, the smallest measurement of time was recorded in quarter hours, with the Helani using heartbeats to count anything smaller. It should be noted that Dwarves had adopted the concept of months and weeks from the Elves prior to this.
The Sylvani Calendar
The wise ones of the Sylvani keep the passing of time with the seasons and cycles of the moon. Years are not counted as significant in any way and not kept track of numerically. Significant events to the Sylvani people are remembered as being a number of lifetimes ago, so the sacking of the Lunar Pool by the Eternal Empire in 5120 AA is approximately 4 lifetimes ago (assuming a Sylvani lifetime is around 250 years like their Helani cousins).
Seasons are the most important to the Sylvani as they are prompts for the people to migrate to the next tree village in their seasonal circuit of their lands.
Human cultures have in the main adopted the time keeping systems of the other races depending on their level of contact, with their own local variations. For example the Cernai peoples of Western Etherias use the Dwarven Stone Age as their calendar. While the scholars in the Kingdom of the Isles use a mixture of Elven seasons with Dwarven year dating for.their records.
Evidence of timekeeping has been found in the Beastmen tribes, especially those of the Centauri. This appears to be sporadic and localised to the more advanced tribes.