The UK tax year starts on April 6 because Julius Caesar got his sums wrong when decreeing all the provinces in the Roman Empire should follow the same calendar in 45BCE.
To be fair, Caesar was not far out when he set the Julian Calendar to run for 365 days but his year was 11 minutes and 30 seconds out on how long the Earth took to orbit the sun.
In Roman times, the discrepancy was hardly noticeable and was widely ignored for hundreds of years despite the timing problems that arose.
Confusion over dates meant many Romans in different parts of the empire applied a different date to the same day.
One specific problem was how long a Roman official stayed in office. The term was based on a calendar year with a length that varied from one year to another, which meant someone could overstay in office by invoking the calendar that suited them best.
Across the centuries, those missing 11 and a half minutes started to add up to make the Julian calendar 10 days shorter than the solar calendar.
In 1582, Pope Gregory decided the time difference was too much and introduced the Gregorian Calendar that shortened the year from 365.25 days to 365.2425 days. That cut 10 minutes and 48 seconds off the year and aligned his calendar with the solar year.
Only a few countries switched to the Gregorian Calendar from the start, including Italy, Spain, and Portugal.
Britain carried on with the old Julian system, with the tax year starting on March 25, which was also the New Year’s Day.
In 1752, King George II agreed to change the British calendar to the Gregorian one.
Fear of losing tax revenues
However, the Treasury was worried about losing 10 days of tax revenues when the calendar changed, so moved the tax year end to April 4 and the start to April 5 to avoid the loss.
That worked well for a few decades, but still caused a problem on leap years, which have 366 days instead of 365.
This came to a head in 1800, which was a Gregorian leap year but not a Julian one.
The Treasury stepped in again to stem a day’s loss of tax revenues and moved the tax year forward another day.
In 1800, the tax year officially ended on April 5 and the new tax year started on April 6.
The dates have stayed the same ever since.
Tax years around the world
Around the world, the tax year is matched to the calendar year in most countries.
The tax year starts in July in Australia, Bangladesh, Egypt, Ethiopia, Pakistan, and the USA.
The Commonwealth countries of Canada, Hong Kong, India, Singapore, and South Africa follow the UK tax year.