Regnal year

A regnal year is a year of the reign of a sovereign, from the Latin regnum meaning kingdom, rule.

The oldest dating systems were in regnal years, and considered the date as an ordinal, not a cardinal number. For example, a monarch could have a first year of rule, a second year of rule, a third, and so on, but a zero year of rule would be nonsense. 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.

Regnal years are "finite era names", contrary to "infinite era names" such as Christian era, Jimmu era, Juche era, and so on.

Reckoning in various cultures

In ancient times, calendars were counted in terms of the number of years of the reign of the current monarch. Reckoning long periods of times required a king list. The oldest such reckoning is preserved in the Sumerian king list.

In England, and later the United Kingdom, until 1963, each Act of Parliament was defined by its serial number within the session of parliament in which it was enacted, which in turn was denoted by the regnal year or years of the monarch in which it fell.