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Julian Calendar: Gregorian Calendars
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In many countries the Julian Calendar was used by the general population long after the official introduction of the Gregorian Calendar. Thus events were recorded in the 16th to 18th Centuries with various dates, depending on which calendar was used. Dates recorded in the Julian Calendar were marked "O.S." for "Old Style", and those in the Gregorian Calendar were marked "N.S." for "New Style".
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The Julian Day Number is defined relative to the Julian proleptic calendar so that from noon of October 4, 1582, the last day in the Julian Calendar, there are 2299160.0 days to noon of the Julian Day Number reference time. This count is a result of considering dates that have leap years every 4 years. Noon of the day following Thursday, October 4, 1582, (Julian Calendar) is the first day of the Gregorian reform (designated as Friday, October 15, 1582, in the Gregorian calendar) and is JD 2299161.0. Normally from this date, the Gregorian calendar is used and the Gregorian calendar rules for leap year apply. It is, of course, possible to continue the use the Julian Calendar after the Gregorian reform or the Gregorian Calendar before the Gregorian reform. Using the Julian Day Number avoids this possible confusion but the system is not in common, everyday use by the general public.
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The Julian calendar was in general use in Europe and Northern Africa from the times of the Roman Empire until 1582, when Pope Gregory XIII promulgated the Gregorian Calendar, which was soon adopted by most Catholic countries (e.g. Spain, Portugal, Poland, most of Italy). Protestant countries followed later, and the countries of Eastern Europe even later. In the British Empire (including the American colonies), Wednesday 2 September 1752 was followed by Thursday 14 September 1752. For 12 years from 1700 Sweden used a modified Julian Calendar, and adopted the new-style calendar in 1753, but Russia remained on the Julian calendar until 1917, after the Russian Revolution (which is ... called the 'October Revolution' though it occurred in Gregorian November), while Greece continued to use it until 1923.
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Although most sources date the conversion from Julian to Gregorian calendar for the pair of days October 4./15., 1582, this is in fact only true for countries where the Roman Catholic Church was influential. Other countries hesitated to adopt the new calendar, in some cases for very long times. Turkey, for instance, converted to the Gregorian calendar on January 1, 1927. Therefore, care must be taken in dating historical events to account for country-specific conversion dates. A fairly detailed list of conversion dates for many countries can be in the Explanatory Supplement (see the list of references).
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New Year's Day had been celebrated on March 25 under the Julian calendar in Great Britain and its colonies, but with the introduction of the Gregorian Calendar in 1752, New Year's Day was now observed on January 1. When New Year's Day was celebrated on March 25th, March 24 of one year was followed by March 25 of the following year. When the Gregorian calendar reform changed New Year's Day from March 25 to January 1, the year of George Washington's birth, because it took place in February, changed from 1731 to 1732. In the Julian Calendar his birthdate is Feb 11, 1731 and in the Gregorian Calendar it is Feb 22, 1732. Double dating was used in Great Britain and its colonies including America to clarify dates occurring between 1 January and 24 March on the years between 1582, the date of the original introduction of the Gregorian calendar, and 1752, when Great Britain adopted the calendar.
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According to the Julian Calendar (which was used by the Pilgrims), the New Year started on March 25, not January 1. It ... declared that a year was 365 days, 6 hours long. However, in 730 AD, the Venerable Bede, a monk, discovered that this calendar was off by 11 minutes and 14 seconds. Nothing was done about it, however, and over the next 850 years the calendar kept getting further and further off until it was nearly ten days off in the year 1582. Pope Gregory XII declared in 1582 a new calendar would be used, named the Gregorian calendar, which would fix the problem.
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Gregorian Calendars