A regnal year is a year of the reign of a monarch. It is from the Latin regnum meaning kingdom or rule. Some of the oldest dating systems were in regnal years. A regnal year usually begins on the date of a monarch's accession to the throne.[1] Year one is counted from day one to the end of the first year of a monarch's reign. Then a second year of rule, a third, and so on. They would not have a zero year of rule. It is displayed as an ordinal, not a cardinal number.
Every year of a monarch's reign falls within two calendar years, unless the reign began on the first day of the calendar year.[2] When converting a regnal year in history to a calendar year, this can cause an error of one year.[2] In England, from the 10th century until the late 13th century, the practice was to count the regnal year from the date of coronation. This was usually a later date than when a monarch was proclaimed king or queen. The current practice of dating from the date of accession began in the late 13th century when the heir was out of the country.[1] Henry III of England died on 16 November 1272 which some sources date as the beginning of Edward I of England's rule.[3] His reign actually began when he was proclaimed king at the New Temple on 20 November 1272.[3] Here the date error between sources would be only four days.