regnal year

A regnal year is numbered from the day a monarch came to the throne. English laws and many colonial deeds and court records dated documents this way, sometimes along with the ordinary year, as in "in the 24th year of the reign of our Sovereign Lord George the Second."

Each regnal year runs from the anniversary of the accession, not from January 1, so one regnal year spans parts of two calendar years. To convert, you need the date the monarch began to reign. In the United Kingdom acts of Parliament were dated by regnal years until 1963.