In 1582 Pope Gregory xiii grew tired of the inaccuracies in the existing ‘Julian’ calendar and ordered the calendar to be changed. The Julian calendar named after Julius Caesar had been in place since 45 BC.
Caesar’s calendar differed from the solar calendar by 11½ minutes.
This was not a big problem at the start, however, after 500 years this small inaccuracy had started to build up to 10 days off the solar calendar. With this in mind, Pope Gregory introduced the Gregorian calendar.
The Gregorian calendar reduced the length of the calendar year from 365.25 days to 365.2425, a reduction of 10 minutes 48 seconds per year!
This and a few other tweaks ensured Pope Gregory’s calendar was a much more accurate time keeper. The Gregorian Calendar was then introduced in Italy, Spain, Portugal and what was then the Polish-Lithuanian commonwealth.
The Gregorian Calendar was initially quite a slow burner in the British Empire, it wasn’t introduced until 1752. By then the British calendar was 11 days off the rest of Europe, with this due to increase as time passed, the British knew it was time for a change.
On the old British Calendar the tax year began on March 25 (the old New Year’s Day). In order to ensure against losing revenue it was decided by the British Treasury that the tax year, which started on March 25 1752, would be of the usual length (365 days) and therefore it would end on April 4, the following tax year beginning on April 5.
Time passed smoothly and most importantly accurately until 1800. Unfortunately 1800 was not a leap year in the new Gregorian calendar but would have been in the old Julian system. Thus the treasury moved the start of the UK tax year from the April 5 to the April 6 and it has remained there ever since!