Provided by Dr. Patrick Woodland

Patrick writes:

You were asking about calendar changes and I have scanned this description 
for you from David Hey, The Oxford Companion to Local and Family History 
(Oxford 1996), p. 63.  I think it gives the basics quite clearly: those of us 
in Blighty currently struggling with our self-assessment tax returns may find 
it of passing interest to know why our annual torment begins on 6 April - it 
is the old quarter day of Lady Day (25 March) adjusted by the necessary 11 
days in 1752! (As an additional chuckle my spellchecker has just queried 
'self-assessment' and suggested 'self-abasement'; yes, dear PC, before the 
Inland Revenue do we all tremble!).

Patrick

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EXTRACT:

Calendar. In 1752 Britain changed from the inaccurate Julian calendar to the 
Gregorian calendar, which had been used in Catholic countries since 1582. The 
eleven days' discrepancy between the two calendars meant that 2 September 
1752 was followed after the change by I4 September. At the same time the old 
custom of starting the official year on Lady Day (25 March), which the Book 
of Common Prayer 'supposed to be the first day upon which the world was 
created and the day when Christ was conceived in the womb of the Virgin 
Mary', was abandoned in favour of the year beginning on 1 January.
 
As 1 January had long been regarded by most people as the true beginning of 
the year, regardless of official reckoning, dates between 1 January and 25 
March were often written as e.g. 1688/9. To avoid confusion, it is a useful 
practice for local and family historians to follow this convention and to 
refer to dates before 1752 as Old Style and those after 1752 as New Style. In 
many places annual fairs and other customary events continued to be held on 
the Old Style date long after 1752. A curious survival of the old system is 
the starting of the official financial year on 6 April, which is Old Lady 
Day, eleven days after the present 25 March.
 
Before 1752 September, October, November, and December were often referred to 
in written records as 7ber, 8ber, 9ber, and 10ber. Quakers called all the 
months by numbers; before 1752 March was their First Month, afterwards it was 
the Third Month. This Quaker practice continued into the 20th century.
 
In the Middle Ages documents were dated not by calendar years but by regnal 
years starting from the time that a monarch ascended the throne. In the early 
modern period documents give regnal years alongside calendar ones. 
 
Addendum:

Other countries had to make different adjustments.
The Russian revolutionaries changed the calendar there in 1917, I think by thirteen days. This is because the old Julian calendar had fallen further behind the Gregorian one since it did not make such an accurate approximation to the actual time taken for the earth to orbit the sun (approx. 365 and a quarter days). The Gregorian gets closer by using leap years, but there are some additional accumulated seconds needing further fine-tuning.  That is where my memory fails me, but there is some sort of formula about leap years ending in 00. It is something like 2000 is a leap year because 20 can be divided exactly by 4, but 1900 was not because it cannot.  That way what would usually be a leap year is treated 'normally', so that the dateline does not get ahead of the actual orbit of the earth around the sun.  If anyone can clarify that bit, I'd be appreciative!

I think the problem about matching births and marriages comes when, say, you 
get a couple who marry in September 1700, but have a child baptised in 
February 1700.  In fact that year 1700 is running from what we would see as 
25 March 1700 to 24 March 1701, so the child is actually baptised in what we 
would call February 1701.  In other words, it was born in wedlock, not 
outside, as might appear at first glance.  This is why one sometimes sees 
references to February 1700/1.  Diplomatic records often go further and give 
both old style and new style dates, e.g. 2/13 February 1700/1 (in other 
words, 2 Feb. 1700 in Britain which was still using the Julian calendar, but 
13 February 1701 on the continent which had adopted the Gregorian calendar).

Incidentally, there was reputedly a considerable public clamour against the 
changes in 1752 with crowds chanting 'give us back our eleven days' in the 
mistaken belief that, as they were destined to die on a particular date, the 
authorities were robbing them of 11 days of their lives. If anyone can get 
hold of it, the December 1999 issue of HISTORY TODAY magazine has an article 
by Robert Poole called 'making up for lost time', which discusses the myth 
versus reality of the 1752 calendar riots (he provocatively suggests that 
they are simply a figment of historians' imaginations!).