Perpetual Calendar Remarks

• The Perpetual Calendar can only be used for years A.D. 1 to A.D. 2400. B.C. years are omitted because it is unknown which years between 45 B.C. and 9 B.C. were leap years. Before 45 B.C. years didn't even have 365/366 days.

• The Perpetual Calendar uses the A.D. (anno Domini) and B.C. (before Christ) year numbering notation. The B.C. notation is probably introduced in the late 8th century. Sometimes one uses B.C.E. (before common era) instead of B.C. and C.E. (common era) in stead of A.D.
  
  A year number notation that doesn't need the A.D. and B.C. suffixes but instead uses negative and positive numbers was introduced in A.D. 1740.
  In this notation a year 0 was also introduced.
  
  

  -2	3 B.C.
  -1	2 B.C.
  0	1 B.C.
  1	A.D. 1
  2	A.D. 2

• The Perpetual Calendar uses the A.D. numbering notation for all years. In real life during the years A.D. 1 to A.D. 283 people numbered the years as 754 a.u.c. to 1036 a.u.c. (ab urbe condita). This numbering notation has a starting point 1 a.u.c. = (around) 735 B.C. (remember: before 45 B.C. years didn't have 365/366 days).
  In A.D. 284 the Diocletian year numbering notation was introduced.
  In A.D. 525 the A.D. notation was introduced. It took centuries before it became the common notation.

• The Perpetual Calendar uses the original introduction date of 15 October A.D. 1582 as the start of the Gregorian Calendar. In real life from A.D. 1582 to A.D. 1918 many people kept using the Julian Calendar for short or long periods.

• The Perpetual Calendar uses the seven day week as we now know it. In real life from A.D. 1 to at least A.D. 321 people used a eight day week.

• The Perpetual Calendar counts A.D. 4 as a non-leap year. In real life in A.D. 4 it probably was a 365 day year.

• The Perpetual Calendar uses 1 January as the starting day of a new year. Since 153 B.C. (or maybe even 610 B.C.) people are using this starting day. An important exception was England that introduced 1 January as the starting day in A.D. 1752.

• The Perpetual Calendar uses 31 days for August, October, December; 30 days for September, November and 28/29 days for February. In real life from 45 B.C. to about A.D. 8 people probibly used 30 days for August, October, December; 31 days in for September, November and 29/30 days for February.

• The Perpetual Calendar uses the name August for the 8th month. In real life from 45 B.C. to about A.D. 8 people used the name Sextember.