Year 0 – the first year after the Unity pact was signed.
Unity of Sime and Gen Territories was formed by the First Contract. It was made between the Tecton, which was at the time only the organization of Householdings, and the out-Territory Gens to avert Zelerod's Doom.
The event of the First Contract was seen as so significant even at the time it happened that a New Calendar was instituted starting with the Year 0 -- a Year between the signing of the contract and the implementation of the new legal structure.
Prior to the Year 0, any calendars kept were local and of the form "In The Time Of (insert local ruler)." After Unity, historians and archeologists added the "minus" counting back from the Year 0 to establish dates for these local calendars.
For the pre-history and history of Sime~Gen, see The Chronology