As with localities an experienced genealogist has usually acquired a customary way to record the dates so as to clearly identify the time where the events of interest occurred. In modern times dates are based on the Gregorian calendar system. This calendar was introduced in various places in Europe in 1582. In Great Britain and its colonies the new calendar was not instituted until 1752, and in Scandinavia, not until the early 1700’s.
Another type of uncertainty with respect to western dates is the beginning of the new year. The ecclesiastical year began on Lady Day (March 25) or in Scandinavia on the first Sunday in Advent. The civil year began on the first of January. Ambiguity is usually avoided by recording both versions of the year for dates between the two days.
Often the initials N.S. (New Style) or O. S. (Old Style) will clarify which calendar was in use independent of the time of the event. One source of confusion was the introduction in the late 18th Century of the special calendar of the French Revolution. All these calendar systems may be described through the use of a GRAMMAR to parse out the pieces of the date, i.e., analyze and classify them.