If you have done English research, you probably know that England adopted the Gregorian calendar in 1752. But do you know when Scotland adopted it? Some say 1600; others say 1752. Which is right?
The Julian calendar, which began the year on March 25th, had been used throughout the Christian world since ancient times. But by the 16th century, scientists agreed that the old calendar improperly calculated the length of a year and was therefore falling gradually behind. During the reign of Pope Gregory XIII, a new calendar was created, called the Gregorian calendar. The new calendar, which began the year on January 1st, was adopted by most of Europe in 1582, but England and Henry VIII had broken with Rome and therefore did not adopt the new calendar.
Scotland, however, was not yet united to England in 1582. The Scottish government decided that January 1st made a better New Year's Day than March 25th, so the decision was made to make January 1, 1599 the new January 1, 1600. As a result, 1599 had only nine months. But that was as far as the change took place in Scotland.
The other change that took place when Europe adopted the Gregorian calendar in 1582 was that 10 days were dropped from the calendar. But Scotland did not follow suit, so year after year in Scotland, as in England, the Julian calendar fell gradually further and further behind. By 1752, Scotland had been (reluctantly) united to England, so when England changed to the Gregorian calendar that year and dropped 11 days, Scotland did the same.
So you decide for yourself. When did Scotland adopt the Gregorian calendar? I say 1752!
For further reading and a document sample, see the Days and Dates article in the scan.org.uk website. See also the Calendar Changes section in the Scotland History article in the FamilySearch Research Wiki.