Between October 15, 1582 and September 13, 1752 Gregorian dates inclusive (corresponding to October 5, 1582 to September 2, 1752 Julian-1 dates inclusive), a gradual phasing in of the Gregorian Calendar took place, but due to wide-spread use of the Julian calendars (some of which have a year number change on a day other than January 1), there was mass confusion in communicating the dates between countries that did not use the same version of the Julian calendar, or one that used the Gregorian and the other a version of the Julian calendar.
While many references use the O.S. (Old Style) notation, it was assumed that it was the Julian calendar that used the year number change of January 1. Not so between England and Spain from 1584 until 1752, as Spain switched over to the Gregorian calendar, but not only did England (as well as Great Britian and its colonies) stick with the Julian calendar, it used the Julian calendar where the year number changed on March 25 instead of January 1, resulting in date confusion of massive levels between the two countries. If it was January 11, 1585 in Spain, it was January 1, 1584 in Great Britain. Some calendars in other countries stuck with the Julian calendar with January 1 as its year number change, but a few had the number change day falling on December 25, the day before Easter Sunday, September 1, March 1, or whatever Julian calendar.
So which Old Style calendar are they referring to when I see the initials "O.S."? Who really knows. They never state which Julian calendar with which year number change day used it is when referring to dates of people and events during the years through the 18th century.
Even I wasn't aware of it until I began studying the ancient calendars and discovered that many countries, nations, towns, etc., used many differing calendars before the Gregorian Calendar became the defacto calendar standard of the world as it is in the 21st century.
So with that, here is a developing glossary of the Calendar Codes Used on my websites. Some people, such as George Washington, the first President of the United States of America, have their original birthdate translated into more than one calendar style birthdates. Washington was born on February 11, 1731 in Bridges Creek, the Colony of Virginia, in British America, decades before the United States declared independence in 1776 and it became the present State of Virginia, U.S. Since England used the Julian Calendar that used the year number change of March 25 up through the end of 1752, it was known as the Julian-Lady Day (or Julian-3-25) calendar where it didn't become the year 1732 until that day. If using the Julian-1 (where the number year change fell on the traditional January 1 day) calendar, Washington's date for that calendar would be February 11, 1732.
Since both of these Washington birthdates are Old Style, which Old Style is what? To end the confusion, I have implemented this method for such dating.
The date of February 11, 1731 J-LD (Julian Calendar, new year falls on March 25)...
is equivalent to...
the date of February 11, 1732 Ju-1 (Julian Calendar, new year falls on January 1)...
which in turn is equivalent to...
the date of February 22, 1732 N.S. (Gregorian Calendar, new year falls on January 1)...
and all of these three dates refer to...
the Julian Period Day of 2361017.
All three of these dates refer to the same date in history, whichever it may be called according to other calendars.
Also adding to the confusion is Sweden from 1700 to 1712 which was trying to gradually convert from the Julian to Gregorian calendar but ending up abandoning the plan when their calendar idea as so unique that it caused date conversion confusion between it and all other countries.
Also when a country decides to move the year number change day back, it results in two dates that are alike for a country, resulting in more confusion. It happened in Normandy the day after December 24, 1066, when the number change day was December 25, when it would have become the year 1067. When the 1066 year began, it began with December 25, 1066 Julian+CD (Julian-Christmas Day, Julian-Nativity Day, Julian-ND, or simply Julian+12-25 all may be used to refer to that calendar) and would end the day after December 24, 1066 Julian+CD. When King William I was crowned King of Normandy, he immediately changed to the Julian-1 calendar, effective December 25, 1067 Julian+CD, which became December 25, 1066 Julian-1. Seven days later, it was January 1, 1067 Julian-1.
So when you see O.S. in other sites, it refers to a version of the Julian calendar, but it's not known which Julian calendar with which year number change day is used. On the Megabirthdays site, O.S. refers to a version of the Julian calendar like the other sites, but we emphasize that it's an unspecified year number change day style.
Then we list "U.S." for Unknown Style as I am not sure which date it corresponds to what calendar.
So here's a partial glossary of the terms used for the Megabirthdays sites.
U.S. = unknown calendar style
N.S. = Gregorian Calendar aka Gregorian-1 or Gregorian-Christ-1
O.S. = Unspecified Old Julian Calendar Style
J+CD = Julian Number Change Day December 25 in year before Ju-1
Hbrw = Hebrew Calendar (uses special month code method)
Ju-1 = Julian Number Change Day January 1
J-LD = Julian Number Change Day March 25 in same year as Ju-1
Ju-3 = Julian Number Change Day March 1 in same year as Ju-1
Ju+9 = Julian Number Change Day September 1 in year before Ju-1
NS-W = Gregorian calendar west of the UTC but east of the IDL
NS-E = Gregorian calendar east of the UTC but west of the IDL
J-Sw = Julian-Sweden Calendar 1700-1712 A.D.
Romn = Date on the Roman Calendar (in relation to the beginning of the month translated from the Kalends countdown format) 753 through 46 B.C.