The Triple Assessment (Income Tax) 1798
William Pitt the Younger (1759–1806)
In 1799, the Prime Minister of great Britain William Pitt enacted the first income tax, and he followed the pattern of the Window Tax by establishing a fiscal year that began on April 5th.  That meant a year that started on April 6th once more, and has remained thus ever since. Addington’s Income Tax Act of 1803, for example, remained to apply “from” 5 April—in this case, from 5 April 1803.  This was a year that began on April 6, 1803 and ended on April 6, 1804.
During a brief respite from the protracted war with France, the income tax was briefly removed in 1802. The statute that eliminated the tax included a clause that allowed tax debts from previous years to be collected. Pitt’s tax year ended on April 5th, according to this saving provision:
Provided, always, and be it enacted, that the said respective Rates and Duties…shall continue in force for the purpose of duly charging to the said Rates and Duties all Persons… who shall not have been respectively charged to the said Duties for the Year ending on the fifth Day of April 1802, or any prior year…
Income tax legislation did not regularly utilize a charge formulation similar to the one advised today by the Office of the Parliamentary Counsel to designate the income tax year until 1860. For the years 1860–61, the tax was imposed “for a year beginning on April 6, 1860.”
For the first time, Section 48(3) of the Taxes Management Act 1880 defined the income tax year and utilizes “from” in the contemporary sense:
Every assessment shall be made for the calendar year beginning on the days herein stated and ending on the days herein specified.
(3) In the case of income tax, from the sixth of April to the fifth of April inclusive in the United Kingdom and Ireland.
Section 28 of the Finance Act of 1919 introduced a new abbreviation for the tax year:
The year of assessment beginning on the sixth day of April 1919 is referred to as “the year 1919–20,” and any phrase referring to two years in the same way refers to the year of assessment beginning on the sixth day of April 1919.
Finally, under Section 4 of the Income Tax Act of 2007, a new term arose as a result of a study aimed at simplifying tax legislation:
(1) A year’s worth of income tax is levied only if an Act mandates it.
(2) A “tax year” is a calendar year in which income tax is levied.
(3) A tax year starts on April 6 and concludes on April 5 the following year.
(4) “The tax year 2007–08” refers to the fiscal year that began on April 6, 2007. (and any corresponding expression in which two years are similarly mentioned is to be read in the same way).
The Federal Reserve Act (1913).
The Law Book: From Hammurabi to the International Criminal Court, 250 Milestones in the History of Law (Sterling Milestones) Hardcover – Illustrated, 22 Oct. 2015, English edition by Michael H. Roffer (Autor)