During the New Day era, the Resurrection Calendar was adopted in order to accurately fix dates. No precise date for the start of the Age could be agreed upon, nor could anyone accurately estimate exactly how much time had passed since The Death of Hope. Indeed, so much time had passed than any effort would be futile.
In what would later be known as N.D. 76, the "Resurrection Calendar" was created, which presupposed that Hunter Jusenkyou conquered death on the twenty-seventh day of the first month of spring, a holiday already celebrated by many branches of the Cardinal Clerics as "Resurrection Sunday". That specific date is also known as "the day hope returned".
The proposed calendar placed this event in year 0, making the first day of the new Alliance "The first day of the first month of the first year of the Age of the New Day". This also allowed them to fix dates within the Long Night, and led to the declaration that the date of Hunter's return occurred in the 3,115th year of the Long Night. The accuracy of these dates is much debated.
Also of note is that the new Alliance did not coalesce as a recognizable force until at least N.D. 43. Placing Unification earlier was held to give the Alliance greater legitimacy by making it appear older on paper. The date was agreed upon by the other major powers as it gave them the ability to fix certain important dates within the Age of the New Day.