In 46 B.C., Julius Caesar established the use of a calendar which came to be known as the Julian calendar. This calendar was calculated with three years of 365 days, followed by a fourth year that contained 366 days, based on an estimation of 365.25 days in a solar year. In 730, St. Bede the Venerable declared that the estimation of a 365.25 day year was 11 minutes, 14 seconds too long, which resulted in a cumulative error of a day about every 128 years. By 1500 the calendar had accumulated 10 days too many. Thus the calendar year was not synchronized with the solar year relative to the vernal equinox.
In 1582, Pope Gregory XIII introduced a correction to the Julian calendar which came to be known as the Gregorian calendar. Pope Gregory ordered ten days to be dropped from October, thus restoring the vernal equinox at least to an average of the 20th of March. This correction is retained by using a leap year of 366 days every four years, except for century years not divisible by 400. This calendar prevents the accumulation of extra days, maintaining closer synchronization with the solar year.
Until the adoption of the Gregorian calendar, March 25th was the first day of the new year. Eventually this was changed to January 1st. This change is reflected in the names of the months September, October, November, and December. Their latin roots mean respectively "seven", "eight", "nine", and "ten," reflecting their original correspondence with March. When the first of the year was changed to January, they became the 9th, 10th, 11th, and 12th months, but kept their original names. The Julian calendar was called Old Style (O.S.) and the Gregorian calendar was called New Style (N.S.). New Style years began January 1st, while Old Style years began usually March 25th.
In America, as in England, the legal year began March 25th, while the historic year began January 1st. During the period January 1st to March 24th, when the date of the legal year differed from the date of the historic year, it was common practice to use "double dates." For example, 20 Jan. 1686/7 would indicate the end of 1686 in the legal year and the beginning of 1687 in the historic year. In this case, 20 Jan. 1686/7 would usually indicate the year 1687 in the Gregorian calendar. The English and their American colonies finally adopted the Gregorian Calendar in the middle of the eighteenth century. To make the necessary corrections, 11 days were dropped. The last day of Old Style was Wednesday, September 2, 1752. The next day - the first day of New Style - was Thursday, September 14, 1752.
In this database, I have simplified dates that fall within these parameters to reflect the Gregorian date. For example, 20 Jan. 1686/7 is rendered 20 Jan. 1687. I have simplified these "double dates" as a matter of preference. If you encounter a date such as 20 Jan. 1687 in this database, it is usually safe to assume that it is a simplified rendition of the double date 20 Jan. 1686/7.
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