Blog: The Calendar and the Tax Year

First published: 11 November 2025, last updated 07 December 2025.

His Majesty's Revenue and Customs has had another of its bright ideas. To make life easier for the self-employed and landlords, from April 2026 they will have to report income and expenses quarterly rather than annually. This has to be done digitally rather than by filling in a form on the HMRC website.

HMRC says, ‘Using software to keep digital records and make regular updates has been shown to reduce the potential for error and time spent making corrections, and thus support business productivity.’ and ‘MTD for ITSA is the cornerstone of HMRC’s commitment to modernising the way small businesses handle their tax and reducing tax lost to error.’

In reality, most people will continue to use their current systems of record keeping and simply use ‘bridging software’, a glorified spreadsheet, to make the submissions. The new system will add cost and administrative burden for no apparent benefit.

One of the complications of this is that the tax year runs from 6 April to 5 April the following year, rather than a convenient date like the start of a month or 1 January. This introduces problems for people who already make quarterly VAT submissions, because these do align to month ends. HMRC has dreamt up a fudge to get round this, by saying that any date from 31 March will be taken as 5 April.

To understand how this complication arose, it is necessary to go back to the calendars of ancient Egypt and Rome to find out how the calendar used today came about.

As always, if you have any comments or questions, please don't hesitate to get in touch via my Contact Me page.

The Calendar

The calendar used today has its origins in Rome and Egypt.

Legend says that in 735 BC, Romulus, the first King of Rome, defined a calendar of ten months. The first months were given names, but then the ideas ran out and the rest were simply numbered.

Month	Name	Meaning
1	Martius	The stormy month, named after Mars, the god of war.
2	Aprilis	From aprire, meaning to open, referring to flower buds opening.
3	Maius	After Maia, a goddess.
4	Junius	After Juno, the queen of the gods.
5	Quintilis	The fifth month.
6	Sextilis	The sixth month.
7	September	The seventh month.
8	October	The eighth month.
9	November	The ninth month.
10	December	The tenth month.
The First Roman Calendar

The year began on the first of March and had 304 days.

Romulus' calendar didn't match the solar year, so his successor, King Numa, added two months before March; Januarius, after Janus, the Roman god of beginnings, endings, and doorways, and Februarius, after the Februa festival of purification and cleansing, bringing the total number of days to 355.

This calendar was used, with extra days and months being inserted to bring it into line with the seasons, until 46 BC, when Julius Caesar ordered sweeping reforms, introducing the Julian calendar, which took effect on 1 January 45 BC.

Egypt

Caesar based his calendar on a system introduced in Egypt by Ptolemy III in 238 BC, which he learned about during his time in Egypt.

At the end of the last ice age, north Africa was a savannah. Climatic changes dried out the savannah and turned it to desert around 7,000 years ago, forcing hunter-gatherers to occupy the fertile Nile valley, where the river provided irrigation, and to become farmers.

The inhabitants of the Nile valley soon recognised that the Nile flooded at the same time every year, which was important for farming, so they counted the number of days between the annual floods and found that there were about 365. By around 2,500 BC, the Egyptians had developed a civil calendar of 365 days, divided into 12 months of 30 days plus 5 additional days at the end of the year.

The Egyptians were aware that, over time, their calendar drifted relative to the seasons and the annual Nile flood, so the true solar year was slightly longer than 365 days. The difference is about six hours, which means that after one year, a calendar of whole days is about 6 hours behind the astronomical reality. After two years, it's about 12 hours behind, after three years it's about 18 hours behind, and after four years, it's accumulated a full 24-hour day of ‘lag’.

In 238 BC, Ptolemy III attempted to address this systematically by decreeing that an extra day should be added to the calendar every four years. This was largely ignored, but would later inspire Julius Caesar's calendar reform.

The Julian Calendar

Caesar moved the start of the year to the first of January. He changed the length of the months, except for February, to alternating 31 and 30 days starting in January. In a normal year, February had 29 days, and every fourth year, called a leap year, 30 days. This gave the years an average of 365¼ days over the four year cycle.

The names of the months remained the same, even though the change of the start of the year to January meant that the six numbered months were now two out, e.g. December became the twelfth month instead of the tenth. The Senate later renamed Quintilis to Julius in his honour. The year then looked like this.

Month	Name	Meaning	Days
1	Januarius	After Janus, the Roman god of beginnings, endings, and doorways.	31
2	Februarius	After the Februa festival of purification and cleansing.	29 or 30
3	Martius	The stormy month, named after Mars, the god of war.	31
4	Aprilis	From aprire, meaning to open, referring to flower buds opening.	30
5	Maius	After Maia, a goddess.	31
6	Junius	After Juno, the queen of the gods.	30
7	Julius	After Julius Caesar.	31
8	Sextilis	The sixth month.	30
9	September	The seventh month.	31
10	October	The eighth month.	30
11	November	The ninth month.	31
12	December	The tenth month.	30
Julius Caesar's Calendar

This worked well, with the easy to remember pattern of month lengths alternating between 31 and 30, until the Senate decided to honour Julius Caesar's successor Augustus by renaming the month Sextilis after him. This wouldn't have been a problem, had the Senate not also decided that Augustus' month couldn't be shorter than Julius Caesar's, so they changed its length to 31 days.

This added an extra day to the year, so February was reduced from 29 days in a normal year and 30 days in a leap year to 28 days in a normal year and 29 in a leap year. To maintain the pattern of alternating month lengths instead of having three months with 31 days in a row, the lengths of all the following months were swapped around. The year then looked like this.

Month	Name	Meaning	Days
1	Januarius	After Janus, the Roman god of beginnings, endings, and doorways.	31
2	Februarius	After the Februa festival of purification and cleansing.	29 or 30
3	Martius	The stormy month, named after Mars, the god of war.	31
4	Aprilis	From aprire, meaning to open, referring to flower buds opening.	30
5	Maius	After Maia, a goddess.	31
6	Junius	After Juno, the queen of the gods.	30
7	Julius	After Julius Caesar.	31
8	Augustus	After Augustus, Julius Caesar's successor	31
9	September	The seventh month.	30
10	October	The eighth month.	31
11	November	The ninth month.	30
12	December	The tenth month.	31
The Julian Calendar after renaming Sextilis to Augustus

This is the origin of the confusing month lengths that people still struggle to remember today.

The Julian calendar was used throughout the Roman empire. When Rome withdrew from Britain, the Christian church influenced many aspects of civil administration. The start of the year was changed from the first of January to Lady Day, the twenty fifth of March, the most important of the quarter days.

Lady Day commemorates the Annunciation, Gabriel’s announcement to Mary that she would bear Jesus, which was considered to be the moment of the Incarnation, and a suitable day to mark the beginning of the year. Lady Day was adopted as the start of the year for legal, fiscal, and administrative purposes. Lady Day was also an important day in the agricultural calendar, before ploughing and sowing began. If tenant were to change farm, Lady Day was the day they moved, and many leases began on Lady Day.

The Gregorian Calendar

The Julian calendar, with the year length made an average of 365¼ days by inserting a leap day every fourth year, was pretty accurate, but the true solar or tropical year, determined by the Earth's revolution around the Sun, which causes the cycle of the seasons, is about 365.2422 days. This is shorter than the Julian year by roughly 11 minutes 14 seconds.

That difference adds up: Every 128 years, the calendar drifts one day later relative to solar time, so seasonal events appear to occur earlier.

This meant that the date of Easter, which is determined by the date of the spring equinox, steadily slid earlier relative to the solar year. Easter is celebrated on the first Sunday after the first full moon on or after 21 March, which was supposed to be the date of the spring equinox. By the 16th century, the spring equinox was occurring around 11 March.

To fix this, Pope Gregory XIII introduced the Gregorian calendar, dropping 10 days to realign the equinox with 21 March, its approximate date at the time of the Council of Nicaea (AD 325). Thursday 4 October 1582 was followed by Friday 15 October 1582.

Gregory also refined the leap-year rule so that century years are leap years only if divisible by 400. This was the only change he made to Caesar's calendar.

Lady Day and the Tax Year

Most of Catholic Europe adopted the Gregorian calendar in 1582, but Britain (and the wider Protestant world) did not do so until 1752. When Britain adopted the Gregorian calendar, Parliament decided that the civil year would begin on the first of January, to align with most of Europe.

When the Gregorian calendar was adopted, the small error in the Julian calendar had accumulated to the extent that eleven days had to be dropped, with Wednesday 2 September being followed by Thursday 14 September. This caused confusion, with some people thinking that their lives had been shortened by eleven days.

However, the start of the financial year in Britain didn't change, and still reflects the older system. Before the change to the calendar, taxes were collected on Lady Day, but merchants objected to tax being collected after less than a full year had passed, so the tax year was altered to begin on 6 April, which is 25 March plus the 11 days that had been dropped.

MTD for ITSA

From April 2026, British self employed traders and landlords will have to make quarterly submissions of income and expenditure under a scheme called Making Tax Digital for Income Tax Self Assessment (MTD ITSA), although the reason for this has not been made clear. No compelling rationale has been publicly stated for requiring quarterly rather than annual submissions, which imposes additional administrative burdens on landlords and the self-employed without obvious benefit.

If a business is VAT registered, it already has to make quarterly returns. These are based on whole calendar months, but the tax year runs from 6 April to 5 April the following year, due to a decision taken in 1752. This means potentially keeping two sets of records for VAT and MTD ITSA returns.

HMRC has recognised this and allows VAT periods and MTD ITSA periods to be brought into line. It appears that businesses may alter their 2025 to 2026 accounting year so that it finishes on 31 March 2026, and then start their MTD ITSA and VAT quarters from 1 April 2026. However, HMRC guidance on this at Your accounting period is very badly explained. The statement says that if MTD ITSA returns are based on an accounting year that runs from 1 April, at the end of the year, the period between 1 and 5 April will have to be added in, which is nonsense, because it will already have been reported in the first submission.

Since basis period reform, an accounting year‐end between 31 March and 5 April is treated by HM Revenue & Customs as aligned to the tax year end of 5 April. A fudge due to the change to the end of the tax year necessitated by the ongoing effects of calendar reforms made in 1752.

Winter Solstice and Christmas

The winter solstice has been celebrated for millennia, marking the start of daylight hours lengthening and the return of warmth and light. In Rome, the winter solstice was celebrated as Dies Natalis Solis Invicti — the birthday of the Unconquered Sun.

In 45 BC, when Caesar introduced his changes to the calendar, the date of the winter solstice was around 25 December. The actual date of the solstice drifted earlier because of the small inaccuracy in the Julian calendar, but once the date of celebrations had been established as 25 December, it was fixed forever.

When the church took over the solstice celebrations and turned them into a Christian event, a decision was made to call 25 December the birthday of Jesus and hold a ‘Christ mass’, which became Christmas. The date was chosen because of the existing tradition of celebrations, not because of any known chronology.

There is no historical or biblical evidence for Jesus having been born on 25 December, nor indeed in any specific year. The New Testament gospel of Luke describes shepherds watching their flocks in the fields at night, which suggests spring or autumn, since shepherds in Judaea would not normally have been outdoors at night with flocks in December. And since King Herod, ruler at the time, died around 4 BC, it must have been in 4 BC or earlier. When Dionysius Exiguus, a 6th century monk, devised the Anno Domini dating system in 525 AD, his calculation of the date of an event over half a millennia earlier was off by a few years.

In his 2012 book, Jesus of Nazareth: The Infancy Narratives, Joseph Ratzinger (Pope Benedict XVI) states that the accepted date for Jesus' birth is incorrect and that the Christian calendar is based on a miscalculation by Dionysius Exiguus. Ratzinger explains that:

The monk Dionysius made an error in his calculations by several years when creating the Christian calendar.
The actual date of Jesus' birth was, therefore, several years earlier than commonly believed, likely around 4 to 6 BC.
The Bible does not specify the date of Jesus' birth and offers few definitive chronological clues. The Gospels focus on the theological meaning of the nativity rather than a precise historical timetable.

The book, the third and final volume in his series on Jesus' life, was published under Ratzinger's own name and is not official Papal dogma, meaning it represents his personal scholarly view as a theologian.

The date of the winter solstice is now around 21 December. It is still celebrated as marking the start of daylight hours lengthening and the return of warmth and light and the promise of spring.

If you have any comments or questions, please don't hesitate to get in touch via my Contact Me page.

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