Early Romans used a calendar of only 10 months, leaving 60 days in limbo. Eventually they added two more months, but they did not have the weeks. Ides was the 15th (or 13th in some months). But the Romans still had extra days in their year, which became “feast days” (used by politicians to their political advantage).
Julius Caesar came into power in 46 B.C. and called upon an Egyptian astrologer to straighten out their erratic calendar. So 46 B.C. had 14 months, called the year of confusion! The Julian calendar was based on a year of 365 ¼ days with an intercalary day added every fourth year (the leap year).
Because the Julian calendar missed 11 minutes every year, there was a lag of nearly two weeks 1600 years later—one day every 128 years. Feast days of the Church were off schedule, and Pope Gregory XIII appointed a committee to study calendar reform. The resulting Gregorian calendar was adopted in 1582 by most Catholic European countries
The Protestants reacted with violent objections and did not adopt this new calendar until 1700 while Great Britain and the American colonies waited until 1752.
In American history during the 1600’s ten days were missing. Thus our double date confusion began with the adoption of the Gregorian calendar. Prior to the year 1752 English records were sometimes kept in Old Style (Julian) and sometimes in New Style (Gregorian) and sometimes in both calendar styles.
The first day of the new year on the Julian calendar was March 25th while the modern Gregorian calendar begins the new year on January first. So between January 1st and March 24th is the crucial gap. Let’s use January 24, 1742/43 as an example. It was already 1743 in most of Europe, but still 1742 in the Colonies, so early records are often written as 1742/43.
By 1 March 1699/1700 the time lag had increased to eleven days. We observe George Washington’s Birthday as February 22nd in our Gregorian (also called New Style) calendar. But he was born in 1732 when the Julian (also called Old Style) calendar was still used. Remember that we lost 11 days in 1752. So Washington was really born on February 12, 1732/33 by the Old Style dating. It was still 1732 in Virginia but already 1733 in most of Europe.
A possible confusion involving numbered months arises with certain dates in the “first month” [March] when the year is not double dated. Although most clerks regarded 25 March as the first day of the Old Style year, some used 1 March. But even if all of them had used 25 March, there’s still the question of what year was intended for March dates prior to the 25th when the entire month is numbered as “first” and the year is not double dated. The eighth month, however, was October of the year written–period.