The question of "why does the UK Tax Year end on 5th April? is one lots of people wonder.

Well here is the history and background.

The Julian Calendar

Julius Caesar the Roman Emperior introduced the Julian year in 46BC as a revision to the Roman calendar. The Julian calendar has been used by some countries right up to the 20th Century. However is most countries it has been replace by the Gregorian calendar. The problem lies in that it is difficult in reality to have part of a day. The astromonical calendar (counting time by the movement of planets in the solar system) does not round to exactly a whole day when counting seconds, minutes and hours. Hence the leap year dates back to at least the Roman calendar which counted as year as 355 days plus and intercalary month of 27 days between February and March.

The Gregorian Calendar 

The Gregorian calendar is the most widely used calendar in the world today. It is a reform of the Julian calendar, first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 by papal bull Inter gravissimas.

Years in the reformed calendar continue the numbering system of the Julian calendar, which are numbered from the traditional Incarnation year of Jesus, which has been labeled the "anno Domini" (AD) era, and is sometimes labeled the "common era" (CE), otherwise known as "Christian Era".

The changes made by Gregory also corrected the drift in the civil calendar which arose because the mean Julian calendar year was slightly too long, causing the vernal equinox, and consequently the date on which Easter was being celebrated, to slowly drift forward in relation to the civil calendar and the seasons.

The Gregorian calendar system dealt with these problems by dropping 10 days to bring the calendar back into synchronization with the seasons, and adopting the following leap year rule:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

The Gregorian calendar is divided into months:

In 1752 the UK switched from the Julian calendar to the more widely used Gregorian calendar.

It was calculated that up to 1752, the Julian calendar had fallen 11 days behind the Gregorian calendar since its inception in 46BC. As a consequenct to correct this the day after 2 September 1752 was revised to be 14 Sepember 1752.

Now comes the crunch.

The legal year in the UK ran from 25 March or "Lady Day". Dropping 11 days to change to the Gregorian calendar would have meant the Goverment lost 11 days effectively and that would have meant 11 days of lost tax revenue! So the legal year was adjusted accordingly and adding 11 days to 25 March takes you to 5 April.

Hence the UK tax year now starts on 6 April and has done since 1752. Based on the historical legal year.

Why it remains there is that probably no government since has figured out a way to change to what we now refer to as a Calendar Year (the Gregorian Calendar) without losing tax reverue. If they could increase tax revenue by changing the tax year to be in line with the calendar year no doubt they would change it.

It is probably a bit to difficult for the Government to figure out .