Regnal year
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.