The world didn’t end last month, then, and a new year is underway. Given that most of us were more than a little sceptical about the prophesy, the averted apocalypse was greeted with fairly muted celebrations. Chinese emperors were less modest about marking significant events. Besides amnesties and tax exemptions, they adopted a unique custom: they changed their reign titles.

A new emperor always took a new reign title, the signifier of his legitimacy, in the first month of the year following his ascension. For example, Li Shimin, the second emperor of the Tang dynasty, was formally enthroned in the eighth month of AD626. In the first month of AD627, he began using the reign title Zhenguan. That year was the first year of Zhenguan; AD628 would be the second year of Zhenguan, and so on.

Emperors were a capricious lot, however. Many changed their reign titles every few years, with enormous administrative consequences, including having to replace all calendars. Empress Wu Zetian had 18 titles in her 20- year reign. It wasn’t until the Ming and Qing dynasties that emperors stuck to one title for their entire reign.

The practice of using reign titles was copied by the monarchies of East Asia and Vietnam. Today, only Japan uses them to mark years, and 2013 will be the 25th year of Heisei. The Taiwanese Republic of China calendar and North Korea’s juche calendar use the same principle but don’t refer to reign titles.