Why the British tax year starts on 6 April

It looks like an odd day to start a year. It is the old new year, moved twice: once by a calendar change, once by a leap day the Treasury would not give up.

In the United Kingdom, the personal tax year runs from 6 April to 5 April. Nobody chose that date in a modern budget meeting. It is what is left of a medieval new year after two small adjustments to the calendar.

For centuries, England's legal year began not on 1 January but on 25 March, Lady Day. It was one of the quarter days, when rents and debts were settled, and over time it came to be treated as the start of the financial year.

Then the calendar changed. Britain still used the old Julian calendar, which had drifted out of step with the seasons and with most of Europe. A law passed in 1750 made 1 January the first day of the year from 1752, and closed the gap that same year by simply removing 11 days: Wednesday 2 September 1752 was followed by Thursday 14 September.

The Treasury keeps its days

That left the Treasury with a short year, and a short year means less revenue. So it extended the 1752 tax year, and the next one began on 5 April instead of 25 March. Eleven days later on the new calendar, but the same length of year as before.

The second move came in 1800. Under the Julian calendar every fourth year has a leap day. The Gregorian calendar skips it in most century years, so 1800 had no 29 February. The Treasury counted 1800 as a leap year anyway, for one more day of revenue, and the start of the tax year moved forward again, to 6 April.

When 1900 came, another century year without a leap day, the date did not move again. It has stayed on 6 April ever since.

One more detail makes it neater. Income tax only arrived in 1799, decades after the calendar changed. It did not get a year of its own design. It inherited the one the Treasury was already keeping.

So every spring, people in Britain work to a deadline set by an old quarter day, eleven missing days and one leap day that never happened.