Your researches have produced a marriage settlement signed by one of your ancestors. It says that it was signed on the 15th day of May, 5 Car II, at London in England. What does this mean? Until towards the end of the reign of Queen Victoria (who died in 1901), all Parliamentary statutes and government orders, and many official and private documents, bore the regnal year of the current monarch. Instead of writing 1841, a document would refer to the year 4 Vict. If the document was a will, then since lawyers were paid by the word, the year would be "the 4th year of the reign of our Gracious Sovereign Victoria, by the grace of God, of Great Britain, Ireland, and all her realms and territories, Queen". Twenty-seven words to tell us the year.
To translate a regnal year to a calendar year, you must know when the monarch first took office (the accession date). Victoria became Queen on 20 June 1837, so the period from then to 19 June 1838 is known as 1 Vict. You will note that there is no zero in a reign. This means that you can’t simply add the number to the year. For example, 20 August 15 Vict is not found by simply adding 15 to1837 and getting 20 August 1852. Remember, the year 15 Vict ENDED on 19 June 1852. I find it easiest to add the regnal year to the accession year (e.g. 15+1837=1852) and then look BACKWARDS from the accession day (20 June) to get the correct date. So 20 August 15 Vict is the August date immediately BEFORE 20 June 1852. It must be 20 August 1851. Similarly 17 March 15 Vict is 17 March 1852.
A table of reigns back to 1485 is listed below. Before mid 1 700, legal documents were often in Latin. This means that the sovereign’s name was the abbreviation for the Latin version of the name, Henricus for Henry, Maria for Mary and so on. Fortunately, the English and Latin abbreviations are the same for most sovereigns. The two exceptions are the Stuart kings, James (Jacobus or Jac and Charles (Carolus or Car).
Now we can translate that date, 15 May, 5 Car 11. Charles II acceded on the death of his father, Charles 1, on 30 January 1649. So 5 Car II ended on 29 January 1654. Then 15 May 5 Car II must have been 15 May 1653. So that was the date the marriage settlement was signed.
But was it? Charles I was executed on 30 Jan l649, and his son Charles II was in France at the time. Charles II tried to retake the throne via Scotland, but was defeated at Worcester in 1651. From that time onwards, he lived in the Spanish Netherlands (present Belgium) or the united provinces (present Holland) until welcomed back to England in 1660. In the meantime, Oliver Cromwell and some very rabid anti-monarchists ran the country. No one would have dared, in the political climate of the time, to write 5 Car II on a public document in the London area in 1654. In Bruges or Antwerp, perhaps; but not in London. Have another look at the marriage settlement. It may contain an error in the date - or be a very interesting forgery!