A detailed chronology is the platform on which many brick walls are demolished. Of course, understanding this chronology is often difficult, especially if two modes of computing dates are used in one historic period.
Starting in England from around the 7th century to as late as the 13th century, the year was reckoned from Christmas Day. But, in the 12th century, the Anglican Church began their year on the 25th of March. Over a period of time, the civilian population adopted this calendar. Thus, the civil, legal, and ecclesiastical year were united. However, this produced a change in the "historical" year, which by historians began on January 1st.
To help clarify this dating, a method was developed to add the date of the historical year to that of the legal year. This was necessary when speaking of any day between the 1st of January [starting of the historical year], and the 25th of March [starting of the legal year]. This became written as January 30, 1648-9. The first year being the civil and legal year, the second date being the historical year.
For the genealogist, this becomes confusing when this method is found in deeds, wills, surveys, and other civil or church documents. Just remember that the last figure (year) always indicates the historical year, or the year according to our present time.
The legal year was ordered to commence on the 1st of January by the reformation of the English Calendar by law titled: "Stat. 24 George II. c 23". Thus, dates recorded before 1753 must take these matters into consideration.