In the UK, the financial year ends on 5 April. This is Lady Day (25 March) plus the 11 days lost in the transition from the Julian calendar to the Gregorian calendar in 1752.
“Your observation about the misalignment between the numerical prefixes of the later months and their positions in the calendar is astute. This discrepancy arises from the evolution of the Roman calendar over time.
Original Roman Calendar: • Structure: The early Roman calendar, traditionally attributed to Romulus, the founder of Rome, comprised 10 months totaling 304 days. The year began in March (Martius) and concluded in December (December), followed by a winter period that was not assigned to any month. • Month Names and Numerical Order: 1. Martius (March) 2. Aprilis (April) 3. Maius (May) 4. Junius (June) 5. Quintilis (fifth month) 6. Sextilis (sixth month) 7. September (seventh month) 8. October (eighth month) 9. November (ninth month) 10. December (tenth month)
Introduction of January and February: • Reforms by Numa Pompilius: To align the calendar more accurately with the lunar year, the second king of Rome, Numa Pompilius, is traditionally credited with adding two months: • Januarius (January) • Februarius (February) • Impact on Month Positions: These additions shifted the original months from their positions, making September the ninth month, October the tenth, and so forth. Despite this shift, the original names were retained, leading to the current numerical misalignment.
Renaming of Quintilis and Sextilis: • Quintilis to July: In 44 BCE, following the assassination of Julius Caesar, the Roman Senate renamed Quintilis to July in his honor, as it was the month of his birth. • Sextilis to August: Later, in 8 BCE, the month Sextilis was renamed August to honor Emperor Augustus. To match the length of July, a day was added to August, resulting in both months having 31 days.
Clarifying Misconceptions: • Insertion of Months: The renaming of Quintilis and Sextilis did not involve the insertion of new months but rather the renaming of existing ones. Therefore, this action did not disrupt the numerical sequence of the months. • Retention of Numerical Names: The numerical names for September through December persisted even after calendar reforms and the renaming of months. This retention is why there is a discrepancy between their names and their positions in the modern calendar.
Conclusion:
The misalignment between the numerical prefixes of the months September through December and their positions in the calendar results from historical reforms and the retention of traditional names despite changes in the calendar’s structure. The renaming of months to honor Julius Caesar and Augustus did not disrupt the numerical naming convention; instead, it was the earlier addition of January and February that shifted the original positions of the months, leading to the current nomenclature.”
You can see how this played out in England, a relative latecomer to the increasing year start consensus. Up until the Calendar (New Style) Act of 1750, the year was considered to start on the 25th of March. This means that historical documents in England containing dates in January or February have what we would consider to be the 'wrong' year number on them. Pretty inconvenient, but at least the roman numbered months would have made some sense to a person living in England before 1750.
In practice, the only reason people can consider the tax year as ending on 31st March is because most people have no significant taxable events occurring between then and 5th April.
But if you sell shares that put you over the capital gains threshold, receive dividends, pay into your ISA or even receive interest from your bank account in those few days, these are all considered taxable events. For most people they are a non-issue due to the tax-free allowances on all these classes of income, but if you exceed the tax-free allowance for any of them, it's important you apportion them to the correct year.
on April 15 the taxes on your first quarter of the current year are also due.
the January 15 quarterly estimate is for the last quarter of the previous year, let's call it 2024. April 15 2025 is when you file your tax return for the 2024, but you HAVE ALREADY PAID OR HAD WITHHELD YOUR TAXES FOR 2024.
If you have earned more wages in 2024 than you did in 2023, you will only have to have paid the same as you ultimately owed in 2023 for 2024, and in that case you will have the chance to cough up the remainder on April 15, 2025, but that is nowhere near close to saying that is when you 2025 taxes are "due" as you are claiming.