There is one technical matter affecting dates which needs to be studied in some detail if the genealogist is to understand and properly interpret the Old Style and New Style dates. This is the important calendar change of 1752. Donald Lines Jacobus. This is not the place for a book review, but I will say I've never seen a better explanation of the transition from the Julian calendar to the Gregorian Calendar than the one Jacobus gives.
Few things are as important to the genealogist as dates are.
We go bonkers, that's what. The Gregorian calendar was named for Pope Gregory XIII, so naturally Roman Catholic countries adopted the new calendar in or shortly after it's introduction. Other countries were not quick to follow. Great Britain eventually approved the change for herself and her Colonies by Act of Parliament in 1751. Eleven days disappeared from the calendar forever.