Calendar (New Style) Act 1750
The Calendar (New Style) Act 1750 (c.23) (also known as Chesterfield's Act after Philip Stanhope, 4th Earl of Chesterfield) was an Act of the Parliament of Great Britain. The Act had two parts: first, it reformed the calendar of England and the British Dominions so that the new legal year began on 1 January rather than 25 March (Lady Day); and, second, Great Britain and its Dominions adopted (in effect) the Gregorian calendar, as already used in most of western Europe.

Chesterfield introduced the Bill into Parliament on 25 February 1750/1. It was passed by the Commons on 13 May and received royal assent on 27 May 1751.

Reasons for change
The Parliament held that the Julian calendar then in use, and the start of the year on 25 March, were

Great Britain and the British Colonies
In England and Wales, the legal year 1751 was a short year of 282 days, running from 25 March to 31 December. 1752 began on 1 January. To align the calendar in use in England to that on the continent, the Gregorian calendar was adopted, and the calendar was advanced by 11 days: Wednesday 2 September 1752 was followed by Thursday 14 September 1752. The year 1752 was a leap year so that it consisted of 355 days (366 days less 11 omitted).

As well as adopting the Gregorian rule for leap years, Pope Gregory's rules for the date of Easter were also adopted. However, with religious strife still on their minds, the British could not bring themselves to adopt the Catholic system explicitly: the Annexe to the Act established a computation for the date of Easter that achieved the same result as Gregory's rules, without actually referring to him. The algorithm, set out in the Book of Common Prayer as required by the Act, includes calculation of the Golden Number and the Sunday Letter, which (in the Easter section of the Book) were presumed to be already known. The Annexe to the Act includes the definition: "Easter-day (on which the rest depend) is always the first Sunday after the Full Moon, which happens upon, or next after the Twenty-first Day of March. And if the Full Moon happens upon a Sunday, Easter-day is the Sunday after." The Annexe subsequently uses the terms "Paschal Full Moon" and "Ecclesiastical Full Moon", making it clear that they only approximate to the real Full Moon.

Scotland
Scotland had already partly made the change: its calendar year had begun on 1 January since 1600. The remainder of the act applied equally to Scotland, a part of the Kingdom of Great Britain since the Acts of Union 1707.

Ireland
At the time, the Kingdom of Ireland was a semi-autonomous kingdom in a personal union with the Kingdom of Great Britain. So that the calendar in Ireland would remain harmonised with that of Great Britain, the Parliament of Ireland passed similarly worded legislation as the "Calendar (New Style) Act, 1750".

United States 
The Calendar (New Style) Act 1750 applied to Britain’s American colonies and some British law, including the 1750 Act, is still applicable in some States. When American Independence was declared in 1776, it was not practical for these former colonies to create an entirely new body of American law to replace British law. The practical solution adopted was to continue to apply British law as it stood in 1776 but subject to the proviso that it could be overridden by any subsequent provision of American law.

Florida provides a suitable example: the Territory was ceded by Spain to the United States in 1819. The new State of Florida repealed Spanish law and enacted a provision which said (and continues to say):

James Bryan Whitfield, a former Florida Supreme Court judge, together with others, produced in 1941 a comprehensive list of the relevant measures. This built on earlier work by Missouri. The list includes the key part of the Calendar (New Style) Act 1750.

There is no Federal calendar law.

Other former British colonies 

The Calendar (New Style) Act 1750 applies directly or indirectly in other former British colonies. The Act remains directly in force in Canada.

Early Australian colonial legislation applied British law. Subsequently various reviews have considered the relevance of old British statutes. Australian States eventually repealed British statutes but re-enacted those which remained relevant, such as the Calendar (New Style) Act 1750. For example, New South Wales passed the Imperial Acts Application Act 1969 No 30. Section 5 and Schedule 1 repeals various British statutes including the Calendar (New Style) Act 1750. At the same time section 16 continues the operation of the British Calendar Act by restating key parts and by referring to that Act for the details. Other Australian States passed similar measures.

New Zealand also passed early legislation at various times applying British law. New Zealand undertook a comprehensive review in the 1980s to clarify which statutes still applied, drawing on Australian research, and produced the Imperial Laws Application Act 1988. This did not repeal old British statutes and re-enact relevant measures. Instead section 1(1) says that British statutes listed in Schedule 1 “are hereby declared to be part of the laws of New Zealand”. This includes the Calendar (New Style) Act 1750. Section 4 said that any other British statutes were not part of New Zealand law and so were, in effect, repealed.

Reaction and effect

The "calendar riot" myth

Some history books say that some people rioted after the calendar change, asking that their "eleven days" be returned. However this is very likely a myth, based on only two primary sources: The World, a satirical journal of Lord Chesterfield; and a painting by William Hogarth.

Chesterfield was behind the Act. He wrote to his son, "Every numerous assembly is a mob, let the individuals who compose it be what they will. Mere sense is never to be talked to a mob; their passions, their sentiments, their senses and their seeming interests alone are to be applied to. Understanding have they collectively none." Here, he was boasting of his skill in having the Bill passed through the Lords; the 'mob' in question was his fellow peers.

When the son of the Earl of Macclesfield (who had been influential in passing the Act) stood for Parliament in Oxfordshire as a Whig in 1754, dissatisfaction with the calendar reform was one of a number of issues raised by his Tory opponents. In 1755, William Hogarth produced a painting (and an engraved print from the painting) loosely based on these elections, entitled An Election Entertainment, which shows a placard carrying the slogan "Give us our Eleven Days" (on floor at lower right). An example of the resulting incorrect history is by Ronald Paulson, author of Hogarth, His Life, Art and Times, who wrote that "the Oxfordshire people … are specifically rioting, as historically the London crowd did, to preserve the 'Eleven Days' the government stole from them in September 1752 by changing the calendar".

Thus the "calendar riot" fiction was born. The election campaign depicted concluded in 1754, after a very lengthy contest between Court Whigs and Jacobite Tories. Every issue between the two factions was brought up, including the question of calendar reform. The Tories attacked the Whigs for every deviation, including their alleged favouritism towards foreign Jews and the "Popish" calendar. Hogarth's placard, part of a satire on the character of the debate, was not an observation of actual crowd behaviour.

Legitimate concerns
There were, however, legitimate concerns lest tax and other payments arise any earlier under the new calendar than they would otherwise have done. Consequently, Provision 6 (Times of Payment of Rents, Annuities) of the Act stipulated that monthly or yearly payments would not become due until the dates that they originally would have done had the Julian calendar continued or, in the words of the Act, "[Times of Payment of Rents, Annuities] at and upon the same respective natural days and times as the same should and ought to have been payable or made or would have happened in case this Act had not been made".

This provision applied to defer payment of Window Tax which was a permanent tax. It did not apply to Land Tax which was re-enacted each year (see the next section below). The Land Tax Act for the year from 25 March 1752 became law after the Calendar (New Style) Act 1750 and therefore over-rode the provision governing the time of payment.

Other effects: Why the United Kingdom income tax year begins on 6 April
Several theories have been proposed for the odd beginning of the British tax year on 6 April. The one most commonly found on British tax websites stems from a book published in 1921 by Alexander Philip. In a brief passage Philips says that eleven days were added to the old tax year which began on 25 March. Then from 1753 until 1799 the tax year began on 5 April. He goes on to say that in 1800 a further day was added so that thereafter the tax year began on 6 April. The extra day was added, he says, because 1800 had 366 days in the Julian calendar but only 365 in the Gregorian calendar. He does not say why it was necessary to add another day. Further, he continues, the tax year was not changed when a thirteenth Julian leap day was skipped in 1900, so the tax year in the United Kingdom still begins on 6 April. 

Philip does not give any reason for his view. He does not cite any legislation or other authority.

Jane Frecknall-Hughes, Professor of Accounting and Taxation at the University of Nottingham, takes the same view in a number of articles. The Professor, like Philip, gives no reasons for her view. A good many websites for accountants have repeated versions of these articles.

Other commentators believe the Philip view is wrong and the correct explanation is that the old tax year used to begin on 26 March, the day after the quarter day. Hence, to deal with the omission of eleven days in September 1752, eleven days were eventually added to the start of the tax year and that led to a tax year beginning on 6 April. There was no need to add a further day in 1800. The taxes charged by the year (Land Tax and Window Tax) applied to a year "from" 25 March and, under a legal rule of interpretation dating back at least to Sir Edward Coke's landmark work of 1628, the Institutes of the Lawes of England, this meant the tax year began a day later on 26 March: 

Coke's Institutes were an important source of education for lawyers and editions were published up to the nineteenth century. This why tax acts in the eighteenth century used 'from' 25 March in an exclusive sense to mean a period beginning on the following day. Numerous court cases have arisen because the technical meaning of from a date in acts and documents has been misunderstood. The Office of the Parliamentary Counsel, which drafts legislation today, has published online drafting guidance which urges against the use of the "from a date" formulation because it is ambiguous.

Contrary to popular belief, the British tax authorities did not add eleven days to the end of the tax year which began on 26 March 1752. Some commentators suggest eleven days were added to avoid the loss of tax. Professor Frecknall-Hughes takes this view as did, remarkably, the Inland Revenue in a note issued on the 200th anniversary of income tax in 1999. But the eighteenth century Treasury did not need to add eleven days because the taxes charged by the year captured artificial, deemed income, and not actual income. This was a fixed annual value for Land Tax (the more important of the two) and so much per window for Window Tax. You paid the same regardless of the year length. Window Tax was a permanent tax and its year did not change till 1758 when the tax was recast and the tax year moved by eleven days to run "from" 5 April. That meant a year which began on 6 April because of Sir Edward Coke's 1628 interpretation rule. 

The Land Tax year never changed and continued to run from 25 March. Land Tax was re-enacted each year throughout the eighteenth century until 1798 when it was made permanent. Moreover, the entire code, running to 80 pages, was re-enacted each year until 1798. Hence there was ample opportunity to revise the date on which the Land Tax year began but no change was made. Because of the transitory value of the annual Act, later editions of the statutes which are available online, omitted the Land Tax Acts. However, the last annual Land Tax Act for the year for the year from 25 March 1798 is available online. The National Archives at Kew holds printed statute series which include copies of all the Land Tax Acts. Key Land Tax Act are also available in Why the Tax Year Begins on Sixth April. The 1798 Act uses the standard "from" formula and provided in section 2: 

The following year William Pitt made Land Tax permanent by passing the Land Tax Perpetuation Act 1798. Section 3, for example, refers to "an assessment made in the year ending on the twenty fifth day of March 1799".

When William Pitt introduced the first income tax in 1799 he followed the Window Tax precedent and adopted a year which ran "from" 5 April. That meant, once again, a year which began on 6 April and this has remained the start of the year ever since. Examination of tax acts confirms that there was no change in 1800. For example, Addington's Income Tax Act 1803 continued to apply "from" 5 April—in this case from 5 April 1803. Again, this meant a year beginning on 6 April 1803. Since taxing statutes before and after 1800 used the from 5 April formula to define the year it is clear that a day was not added in 1800. Compare the Window Tax Act 1758 and Addington's Income Tax Act 1803.

The legal analysis is buttressed by accounting practice from time immemorial. A quarter day, such as Lady Day on 25 March, marked the end of an accounting period and not the beginning. This view is taken by leading authorities including The Exchequer Year, A Handbook of Dates, The Pipe Roll Society and Dr Robert Poole in two works. In the 1995 work Calendar Reform Dr Poole cites Treasury Board Papers at the National Archives under reference T30 12 and explains that the accounts carried on being drawn up to the same four days in the real world but the dates had moved on by eleven days. He says:

These were the old quarter days of 25 December, 25 March, 24 June and 29 September plus eleven days. Dr Poole's analysis is confirmed by a minute of the Board of Customs on 19 September 1752, shortly after the omission of the eleven days 3 to 13 September 1752 and not long before the first quarter day affected by the omission—Michaelmas 29 September 1752. The minute says:

It was not until 1860 that tax legislation consistently adopted a formulation of the kind recommended today by the Office of the Parliamentary Counsel to identify the income tax year. For 1860-61 the tax was applied "for a year commencing on 6 April 1860".

Section 48(3) of the Taxes Management Act 1880 provided a definition of the income tax year for the first time:

Curiously the draftsman here incorrectly uses "from" 6 April but still makes the year clear.

Section 28 Finance Act 1919 provided a new shorthand way to refer to the tax year:

Finally, following a review aimed at simplifying tax legislation, a new definition appeared in section 4 Income Tax Act 2007:

Religious dissent

In Appalachia, some Scots-Irish settlers resisted the changeover mandated by the Calendar Act by continuing to celebrate Christmas on January 6 (N.S.), referring to the day as Old Christmas.