International Calendar/Date marker

Marker	Mnemonic	Epoch	Leap cycle	Leap items	Leap rule
none		0001-01-01	400	1/97 d, 1/71 w	4:100:400
		by majority of days
A	alternative, academic; ancient; Armenian	?	?	?	?
		?
B	biblical, Babylonian; Baháʼí; Buddhist	?	?	?	like default
		week starts Sunday
C	civil; Christian, church; Chinese; Celtic; Coptic	?	?	?	?
		?
D	default, date, day	2001-01-01	293?	?	23:22, 28:34, …?
		?
E	Egyptian; Enoch; Ethiopian	?	?	?	?
		?
F	financial, fiscal	custom	400	5–6/2097 d	4:100:400
		360-day year aligned with standard count, week unused
G	Gregorian, global	1582-10-15	400	1/97 d, -02-29	4:100:400
		weeks begin Sunday, the first Sunday of January starts W01, days before that belong to W00, weeks do not span year boundaries, i.e. they can be partial
H	historic; Hijri; Hindu; Hebrew?	?	—	—	none, 365-day year
		years begin -03-01 at any day of the week, the standard week cycle is used
I ≠ 1	international; Indian; Islamic	2001-01-01	293	1/71 d, 1/52 w, 1/13 m, 1/4 q	?
		inherent
J	Julian Jewish	AUC	4	1/1 d	every fourth year a day is added to the last month, i.e. February
		years begin -03-01 at any day of the week, the standard week cycle is used
K	Kalendis	2001-01-01	293	1/52 w	distribution as even as possible, 45:79 sub cycles
		inherent
L	lunar, lunation	?	?	?	?
		?
M	moon, month, Meton, monetary, Muslim	?	?	?	?
		?
N	no leap, null year; new	2001-01-01	293	1/1 y	every 294th year is year zero of the next cycle which then aligns once again with the 293-year cycle
O ≠ 0	old	?	?	?	?
		?
Q	quarter; Qumran?	?	?	?	?
		?
S	season, solar, sun, solstice	?	?	?	?
		W01 contains / starts after Northern winter solstice
U	user	custom	custom	custom	custom
		custom
V	Vedic	?	?	?	?
		?
W	week	?	?	?	?
		?
Y	year	?	?	?	?
		?
Z	zodiac; Zoroastrian	?	?	?	?
		W01 contains / starts after Northern spring equinox ?

The markers ‘P’, ‘R’ and ‘T’ are reserved, as is ‘X’, and must not be used. The letters ‘F’, ‘L’, ‘M’, ‘Q’, ‘S’ and ‘W’ should not be used as date markers unless they are semantically compatible with like markers defined herein.

The year count needs to be compatible with the ±CCYY format. This means, among other restrictions, it may not be shorter than 4 digits, except when the cycle length is smaller than 100 years, because YY may be used then. Note, though, that the year count does not have to be purely incremental, i.e. the last three or four digits may be used for counting years of a cycle and then do not necessarily run through 999 or 9999.