Ages of the World

The longest-view timeline is called the Age of Kinvari, shortened to Kith when listing dates, that begins at the creation of the first Aeldvari. Each millennium is counted as 1 in this timeline. Aeldvari were created in 0 Kith, Duovari in 40 Kith, Mervari in 56 Kith, Hymvari in 78 Kith, and Urkvari in 83 Kith. The Shattering occurred in 86 Kith, and the present is 97 Kith.
The common-knowledge timeline involves a number of epochs, each begun by the creation of one of the prime races of kinvari, with the current epoch the only exception--it began after the Shattering, when the last physical influences of the gods ceased to be visible to mortal eyes after a worldwide arcane catastrophe.
Epochs:

Primal (the period before any kinvari were created), unknown duration

Kith (from Aeldvari creation to Duovari creation) - 40,000 years - 0-40 Kith

Stone (Duovari creation to Hymvari creation) - 38,000 years - 40-78 Kith

Clay (Hymvari creation to Urkvari creation) - 5,000 years - 78-83 Kith

Beast (Urkvari creation to the Shattering) - 3,000 years - 83-86 Kith

Arcane (from the Shattering to the present) - 11,000 years - 86-97 Kith

Upon the event that defines a new epoch, the current epoch is immediately concluded, regardless of where it is within its current millennia. For example, it was in the middle of the 700s in 40 Kith when Duovari were created; however, 40 Kith was concluded without reaching 1,000 years, and the calendar moved instantly into year one of 41 Kith / 1 Stone.
The current year (at the beginning of Tea & Sorcery) is 423 (97 Kith), also written as 423 (11 Arcane) for Aeldvari and Duovari or 423 (11 Common) for Hymvari and Urkvari. When stating dates, the current millennium is not included (unless for extremely specific uses, like contracts or rituals) - the epoch portion of the date is only needed when speaking outside of the current millennium.
The Mervari keep their own calendar, beginning when their race was created and ignoring the creation of the other prime races, much as the others ignore their creation date in measuring epochs. The Siren epoch is 30k years older than the current epoch, since Mervari were created in 56 Kith, ~16k years after Duovari and ~22k years before Hymvari. For them, the present year is 423 (41 Siren).
Below is a table comparing the different epochs' dating systems.