In early British legal documents, the year is often given as the number of years of the king or queen’s reign, rather than the year AD (Anno Domini). It was counted from the monarch’s accession, not their coronation.
Another complication is that until 1752 the English legal year started on Lady Day (25th March), the quarter day marking the Feast of the Annunciation. So, an event between 1 Jan and 25th March could be in one of two years, depending on the context. Except in Scotland, which had different quarter days but (from 1600) used 1 January as the start of the legal year. And prior to the 13th century the English legal year began on Christmas Day, but our research hasn’t gone back that far yet!
And another complication was the use of the Julian calendar. The system (365 days a year, plus a leap year every 4 years) had been introduced by the Romans. But by the 16th century it had fallen 10 days behind the Earth’s celestial position, which worried the Church because Easter was becoming later in the year. Most of Europe switched to the Gregorian calendar (named after Pope Gregory XIII) in 1582, which involved skipping 10 days to catch up with the accumulated error, then missing the occasional leap year. England and Scotland, not wanting to be associated with a papal decree, persisted with the Julian calendar until 1752. By then the difference was 11 days, so at midnight on 2nd Sep 1752 in Britain it became 14th Sep.
However, to get a full 365 days of tax revenues, the British Treasury extended the 1752 tax year by adding 11 days to the original year, so that it ended on 4th April under the new calendar. A further adjustment was made in 1800 when the Julian and Gregorian calendars drifted another day apart. So, in 1800, the British tax year ended on 5th April. No further adjustments were made, so the British (and Australian) tax year still ends on 5th April.
