The Imperial Calendar (commonly abbreviated as IC) is the most-commonly used dating system on the continent of Tian Xia, although it is far from the only one. Other popular calendars include the Mithral Count used by the elves of Jinin, and the Sovereign Scroll of the Dragon Emperors of Xa Hoi. The Imperial Calendar counts as its genesis the founding of Yixing, Tian Xia's first empire. As this occurred exactly 2,500 years before the founding of Absalom, conversion between Absalom Reckoning and the Imperial Calendar is quite easy.
The chroniclers of Tian Xia divide known history into four ages: the Age of Dragons, Age of Ashes, Age of Ascendancy, and the Age of Succession. The Age of Dragons is largely mythical, and predates humanity's arrival on the continent. The Age of Ashes was ushered in by Earthfall; even though that event happened on the far side of Golarion, the effects were felt in Tian Xia as well.
The Age of Ascendancy began in -2499 AR with the founding of the first human empire, Yixing, and lasted until the death of Aroden and the collapse of the last great empire, Imperial Lung Wa, which ushered in the current Age of Succession.