The Valhallan Timeline is measured in years and Aeo's (ie. 200 1AE = 200yrs into the 1st Aeo), an Aeo is a period of time which can only be ended when it is judged that an immense historical alteration or surge has taken place. This must be decided unanimously by four independent parties; the Empirical King (formerly Kriinali King), the Scholar of Arms at the Panemaar Academy, the Grand Mage of Islehart, and the residing Vaniirian King. Note: Prior to the Empires discovery of Vanir in the third Aeo, only the three parties were required - Vanir was included in the pact as a sign of good faith.
If one of these parties is without a leader at the time a new Aeo is proposed, the final decision is passed to The Temple. Such a scenario has occurred only once, in 451 2AE when the Empirical King at the time, Heren-Akhlan Soorvyid the Eighth threw himself off Tyrsea Promontory - that act itself being the final straw that prompted the call for a new age.
PE (Pre-Era) Edit
420 PE: Almost as quickly as they had risen, the Dwemer mysteriously fall, becoming all but extinct by 429 0AE.
2071 PE: The Ecetere form an alliance with the Dark Elves of Black Marsh, forming the Kriinali Dominion - the first unified government of Yjaal. As part of this alliance, a global time scale is agreed upon and the first Aeo begins.
449 2AE: Tensions rise once again between the Ravens and the Dominion after a Kriinali scout cohort allegedly razes a Raven village to the ground. The allegations are never confirmed, but burgeoning peace talks between the two factions dissolve completely as Raven-Queen Tanitha cuts off all ties to the Dominion and refuses any further correspondence.
4 4AE: The Dark Elves elect to peacefully break away from the Dominion, forming their own sovereign nation Elsweyr. The Dominion is thus dissolved and is re-constituted as the Empire, without the inclusion of the Dark Elves. A new Aeo is briefly considered, but due to the predominantly symbolic reasoning for the shift and low diplomatic impact it is deemed unnecessary.