Calendar Years:  The Roman year, inaugurated by Caesar and used by the old empire, began on January 1st, dubbed stylus communis (style of the people) and occasionally stylus circumcisionis, but in the middle ages different locations even started their ecclesiastical year at different times: on December 25th, Christmas day (stylus nativitatis) or stylus curiae Romanae (of the Roman curia), since the papal chancellery sometimes opened that day.  Some, as in England, dated from March 25th (Annunciation Day), or from Good Friday, or the day after, or from Easter.  Still others began their year in March, near the vernal equinox, when some old German calendars and Rome's pre-Julian calendar began.⁵  For example, an English date transcribed 12 Feb 1742/43 likely reflects a Julian calendar date originally written "1742", but is reckoned 1743 by the Gregorian calendar.  The regnum year begins on the date that the monarch (king, pope or even bishop) came to the throne (for Edward III, this was January 25, 1327).  So, if you found an English document dated 6 Jan Edw III:1, this would translate, by modern reckoning, to January 6, 1328.  There is also the agricultural year which begins September 29th (Michelmasse), when tthe accounting done, and new officers are chosen for the manor and for the shire, or county (reeves, sheriffs, etc.).

   Date transpositions and comparisons become more baffling when you allow that different kingdoms used different systems, and one could travel from France to Italy in a week's time and pass through several different years!  January 1st became the general date mark by the end of the 16th C., but the English (and the American colonies) did not move from March 25th until 1752, when they adopted the "New Style" (as Protestants, they refused to call it "Gregorian") calendar, dropping 11 days from the month of September that year.⁴

   The Christian church adopted the Julian (Roman) year, borrowed from the ancient Egyptian solar calendar of 365¼ days (11 minutes and 14 seconds longer than the actual solar year) . . . which eventually led to the reformed (current) calendar instituted by Pope Gregory in 1582, but retained the lunar system, moving some feast days (like Easter) around the solar calendar to keep them in step with lunar cycles.  During the first centuries some Christians dated from the "Indiction" (15 year Roman cycles starting in 312), others from the Roman conquest of Spain in 38 BC, and others from the Era of the Passion (33 years after the nativity).  The Anno Domini system was proposed by the monk Dionysus Exiguus in 525, but was not generally adopted in Europe for some centuries.  The use of B.C. was not used by scholars until the 17th century.⁴

   The Jewish calendar, which begins from a traditional date of creation (Anno Mundi) 361BC, is based on lunar cycles, which use twelve months of 29 or 30 days (about 354 days) and add an extra month every two or three years to account for solar cycles.  The Muslim calendar, which dates from the Hegira, 16 July 622AD, uses the lunar year.⁴

    The Year 2002 was: 1999 according to Christ's actual birth circa 4 B.C.;  2756 according to the old Roman calendar; 5762 according to the Jewish calendar; 1422 according to the Muslim calendar; 1380 according to the Persian calendar; 5122 in the current Maya great cycle; 6239 according to the first Egyptian calendar; 211 according to the calendar of the French Revolution; and the year of the Horse according to the Chinese calendar.⁵

⁴Donald Boorstin, The Discoverers, Random House, New York, 1983

⁵David Duncan, Calendar, Humanity's Epic Struggle to Determine a True & Accurate Year, Avon Books, 1998