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. 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. When converting a regnal year in history to a calendar year, this can cause an error of one year. 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.

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