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…