The meaning of AD is Anno Domini or Year of our Lord referring to the year of Christ’s birth. Today, based on historical evidence relating to Herod and astronomical evidence relating to eclipses and star novas, most historians believe Christ was born a few years earlier. (That’s the reason purists insists the 21st century began January 1, 2001.
It was an improvement upon the Julian Calendar to keep the average length of the calendar year better in line with the seasons. The rules, months, and days of the Gregorian calendar are the same as those of the Julian Calendar, except for the leap year rules.Perhaps the most unfortunately characteristic of this convention is that "BC" is a suffix (used after the year) while "AD" is a prefix (used before the year).This is inconvenient when generating computerized lists because extra columns must be reserved for both prefixes and suffixes.The astronomical year 0 corresponds to the year 1 BCE, while the astronomical year -1 corresponds to 2 BCE.In general, any given year "x BCE" becomes "-(x-1)" in the astronomical year numbering system."The legal code of the United States does not specify an official national calendar.