Some of old editions Burke's Peerage have "living temp Henry II", which is fairly obvious, but what does "living 5 Henry IV" mean, I wondered?
If it meant 5th year of his reign, why not just say 1404?
I discovered that this is the system of regnal years which in medieval times was more commonly reckoned from 25 March (or sometimes from 25 December). In England, and later the United Kingdom, until 1963, each Act of Parliament was defined by its serial number within the regnal year in which it was enacted. In Canada, acts of Parliament are dated by the session, the Parliament, the regnal year, and the calendar year. So, for example, a bill passed in the second session during the period spanning 2007-2008 would be dated thus: Second Session, Thirty-ninth Parliament, 56-57 Elizabeth II, 2007-2008.
Regnal years run from the date of accession of the monarch in question, so if a king inherits the throne on the 3rd September, his first regnal year will run until 2nd September the following year, and so on. The last regnal year of a king runs until the date of his death, when the first regnal year of his successor begins, and so the last regnal year of a king can be of any length.
Until the change of the calendar in 1752, the calendar year was regarded as beginning on 25 March, and dates from 1 January to 24 March are thus recorded as one year earlier than we would regard them now. Definitely a trap for the unwary in birth and marriage dates, as it can sometimes give rise to a false appearance of illegitimacy. If parents were married on 6 April 1744 and their first child was born on 20 March the following year the date of birth would be given as 20 March 1744, old-style.