If you researching back before 1752 care needs to be taken in interpreting the dates on old documents - and even more so on modern documents referring to earlier dates which don't say how the date was originally recorded. It becomes even more complicated if your family research takes you across the English Channel into Europe, as the problems date back to 1582. There is a well-written article "Understanding Julian Calendars and Gregorian Calendars in Genealogy" in the online magazine GenealogyInTime.