The Julian calendar was the calendar originally used by the colonies of British America. The Gregorian calendar eventually replaced it. The difference between the two calendars is that the Gregorian calendar added days and better matched the length of a solar year. Also different, until 1751, March 25 was the first day of the new year. So, the day after December 31, 1749 was January 1, 1749. And the day before March 25, 1750 was March 24, 1749.
What does this mean when researching historical records? A couple of things …
Since March 25th through March 24th are a complete year in the Julian calendar, the same year in fact, a marriage that occurred, let's say on April 1, 1705, followed by a birth of a child in January 1705 appears to be either illegitimate or not correct, when it is neither. The birth occurred after the marriage.
Additionally, records with dates between January and March 24th will sometimes contain this notation: 6 February 1723/4 where 1723 is the Julian year. Record transcribers will often not use the notation and instead choose one date, usually the first year.
So when looking at records with dates between January 1st and March 24th prior to 1751, use common sense when interpreting the record. For example, two children born too close together, or a death prior to a marriage, may not be the case.