Applying this ancient epoch system to modern calculations of time, which include zero, is what led to the debate over when the third millennium began.
In ancient times, calendars were counted in terms of the number of years of the reign of the current monarch.
The Zoroastrian calendar also operated with regnal years following the reform of Ardashir I in the 3rd century.
In China, the continuous use of era names began in 140 BC, during the rule of the Emperor Wu of Han.
Since 140 BC, era names served as titles for the purpose of numbering and identifying years.
As a result of Chinese cultural influence, other polities in the Sinosphere—Korea, Vietnam, Japan, and Ryukyu—also adopted the concept of era name.
[2][3][4] Abolished era names may be reused, for example as a means of claiming or denying political legitimacy.
Hongwu had in fact died in 1398, and the short reign of the Jianwen Emperor, who ruled between 1398 and 1402 was written out of the official record.
The short lived Daxi Kingdom [zh], post Zhang Xianzhong, used the Ganzhi calendar without era names.
[9] The name of his era is Reiwa, which was formally announced by the Government of Japan a month before Naruhito succeeded the throne, on 1 April 2019.
From 1952 until 1961, years were numbered in Dangi in South Korea, counting from the legendary founding of Gojoseon in 2333 BC.
[2][4][3] The titles were adopted in historical Vietnam for the purpose of year identification and numbering.
The system still survives today and is used in the Odia panjis to mark the titular regnal year of the King of Puri, Gajapati Maharaja Dibyasingha Deba of the Puri Estate, whose title carries the legacy of historical ruling monarchs of Odisha.
[14] The regnal years used throughout the Commonwealth realms are identical to one another, as they share the same line of succession.
The present monarch, Charles III, became the sovereign on 8 September 2022, after the death of his mother Elizabeth II.
All Acts are given an individual chapter number, assigned by its numeric order of when it received royal assent, along with the regnal year, and the name of the reigning Monarch of Canada.
While not strictly a regnal year, time in the United States of America can be derived from the Declaration of Independence (July 4, 1776).