Calendar Wiki
Flag of the International Calendar

Flag of the International Calendars, showing 7×4 segments

The International Standard Calendar is based upon the rules laid out in the international standard ISO 8601. It consists of the primary International Standard Month Calendar, which defines a year based on months and a leap day rule (Gregorian calendar), the alternative International Standard Day Calendar, which just places the leap day at the end of the year, and the secondary International Standard Week Calendar, which uses the same year count but subdivides a year into weeks. The International Calendar is a superset thereof that contains more notation variants and auxiliary calendars. For a similar project, see Extended Date and Time Format (EDTF) at the US Library of Congress, which is a part of ISO 8601-2:2019 in slightly modified form.

Both are more notation frameworks than actual calendars, but only certain calendar designs are compatible with the framework and in some instances, out of several possible alternatives, a single one had to be chosen to be supported.

Disclaimer: The use of the term “ISO” in the body of this article is not meant to claim or imply any formal approval or recommendation of proposals presented herein by the International Organization for Standardization (ISO). It is meant simply to signify that the proposals are compatible with ISO 8601.


A modest way to propose calendar reforms is a set of incremental, backwards-compatible additions and clarifications to this standard. Several such enhancements are possible, some of which are furthermore compatible with alternate calendar proposals, i.e. the International Calendar is a superset thereof.

It is a best practice accepted in standardization to collect existing use deviating from the current standard, analyze it and, finally, form rules based upon the findings, which are compatible as much as possible with both the existing standards and popular habits.

There is, for instance, much precedent in labeling quarters of a year “Q1” through “Q4”, although the exact definition of a quarter varies. It is also common to speak of the nth week of a month, but the standard currently only implicitly defines a rule for that by prescribing which year a week belongs to. It is also common to speak of the nth (instance of a) weekday in a month.

Especially for some religious and esoteric purposes, the solstices and equinoxes (including visibility of constellations) or the phases of the moon are more relevant than months and weeks. It may make sense to define notation for auxiliary years based on that, aligned with the common year count. The seven-day week cycle is important to several religious groups and therefore is hard to break apart from, as has been seen by the failed attempt to introduce the World Calendar by the United Nations in the 1950s.

In financial contexts, the month and year are often simplified to 30 and 360 days, respectively. Elsewhere, a month is often thought of as consisting of 4 full weeks only, which would require slightly more than 13 months per year.

The International Calendar is not related to the International Fixed Calendar.


Types of formats

  • There must not be ambiguous formats. If two schemes would result in two or more confusable formats, all of them or all but one must be declared invalid.
  • Only add a redundant format if there are good reasons for it.
  • Extend existing schemes and conventions.
    • Apply week of year determination rule to months, quarters etc.
    • Reuse the ‘W’ convention for other entities if necessary.
  • Single alphabetic letters in a format are called “markers”.
  • Every date format must be able to resolve any day. The day must be the smallest possible unit in a date.

Standard vs. basic

  • The extended format becomes the standard format, the basic format is a condensed or collapsed or compact version thereof.
  • Collapse everything or nothing.
  • Support condensed format where possible.
    • Do not condense formats with a one-digt part, except when it is the last one and follows an alphabetic marker. (This is a suggestion that this page does not yet adhere to.)
    • Do not condense formats with plus or minus sign before the year number.
  • Do not support two-digit years without century and era (YY) in new formats, but consider their existence.
  • Do not support years with more than ten digits which is already more than than the age of the universe.
  • Implementations may support 4 levels of condensation: standard (with all separators present), collapsed (markers consume preceding separator), condensed (all separators only removed if possible) and compressed (all separators suppressed, despite ambiguity ensuing). The standard only describes standard and condensed forms, though.

Implied formats

  • Partial values on the right may be left out. This specifies less specific dates.
  • Partial values on the left may be left out without dropping separators and markers. Missing parts are implied (usually using the live value).
    • Separators may be dropped if markers alone make the format unambiguous.
    • In durations sepcified dby start and end date, omitted fields in the end date take the value from the start date. Both should use the same format, unless agreed on otherwise.

Other rules, requirements, constrictions

  • Do not break the week cycle.
  • Use 97/400 leap year cycle with 4–100–400 rule by default.
  • Dates are ordinal, except for the year, but times are rational, i.e. the former start at “first” (1), the latter begin with “none” (0).


This subsection lists topics that are known to be left to do.
  • Find and correct mistakes.
  • Expand sections marked “under construction”.

Existing formats[]

Existing formats with digits code
level of detail full date sample
c CC 2 2024
y CCYY −4 N/A
+CCYY +4 +2024
 CCYY 4 2024
d/y ±CCYY-DDD ±4-3 2024-205
 CCYYDDD 7 2024205
m/y ±CCYY-MM ±4-2 2024-07
d/m/y ±CCYY-MM-DD ±4-2-2 2024-07-24
 CCYYMMDD 8 20240724
w/y ±CCYY-WWW ±4-W2 2024-W30
±CCYYWWW ±4W2 2024W30
d/w/y ±CCYY-WWW-D ±4-W2-1 2024-W30-3
±CCYYWWWD ±4W3 2024W303

The identifier ±CCYY (±4), on this page, refers to any of the three 4-digit formats for small years above and to any large year as specified in the next section.

General clarifications, additions or enhancements[]

Large years[]

Numeric date format
‘+’ or ‘-’ marker

Note: EDTF has a prefix marker ‘y’ and exponential ‘e’ notation

Proposed year formats with digits code
level of detail full date sample
y ECCYY +ECCYY ±5 +02024
EECCYY +EECCYY ±6 +002024
EEECCYY +EEECCYY ±7 +0002024

±CCYYMM (with leading plus or minus sign) could be confused with six-digit years ±EECCYY, seven-digit and eight-digit years would be ambiguous with the condensed ±CCYYDDD ordinal dates and ±CCYYMMDD full dates, respectively.

Therefore compact formats are only valid without a leading plus or minus sign unless they contain a marker as first character after the year. Note, that the deprecated YYDDD is already compatible with five-digit years ±ECCYY (i.e. almost all of human history).

Todo: Restricted era format with 400-year cycles, e.g. ±EE|YYY

Four-digit years should not have a preceding plus sign to avoid ambiguity. Two digits designate a century, but it is not possible to pad it on the left with zeros, although ISO 8601:2004 allowed this expanded format for prior mutual agreement.


The characters minus sign U+2212 ‘−’ and en dash U+2012 ‘–’ are also valid instead of hyphen-minus U+002D ‘-’ before years. They are invalid as a separator, but the characters hyphen U+2010 ‘‐’ and non-breaking hyphen U+2011 ‘‑’ are valid separators besides U+002D. The character soft hyphen U+00AD ‘­’ is a valid separator, but should not be used due to its default invisibility.

The em dash U+2014 ‘—’ is neither a valid minus sign nor a separator, it has special purposes. All other hyphen, dash and minus characters from Unicode must be normalized to one of the aforementioned in a proprietary manner, which may include them being discarded altogether.

Other decimal digits, e.g. arabic, indic or circled ones, are not directly supported. They should be converted to standard digits ‘0’, ‘1’, ‘2’, ‘3’, ‘4’, ‘5’, ‘6’, ‘7’, ‘8’ and ‘9’ prior to data interchange.

All space characters, varying only in width and breaking behavior, (U+00A0, 2002–200B, 202F, 205F, 3000) must be normalized to space U+0020 ‘ ’.

  • minus ::= [\u2212 | \u2012 | \u002D] => \u2212
  • date-separator ::= [\u2010 | \u2011 | \u00AD | \u002D] => \u2010
  • digit ::= [0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9]
  • space ::= [\u0020 | \u2002 | \u2003 | \u2004 | \u2005 | \u2006 | \u2007 | \u2008 | \u2009 | \u200A | \u200B | \u202F | \u205F | \u3000 | \u00A0] => \u0020

All roman letters used are case-independent and should be normalized to uppercase.


Proposed truncated formats for existing formats with digits code
level of detail century implied year implied more implied sample without century sample without year sample with just day
d/y YY-DDD 2-3 -DDD -3 24-206 -206
YYDDD 5 DDD 3 24 206
m/y YY-MM 2-2 -MM -2 24-07 -07
invalid invalid
d/m/y YY-MM-DD 2-2-2 -MM-DD -2-2 --DD --2 24-07-24 -07-24 --24
YYMMDD 6 invalid invalid 240724
w/y YY-WWW 2-W2 -WWW -W2 24-W30 -W30
YYWWW 2W2 WWW W2 24W30 W30
d/w/y YY-WWW-D 2-W2-1 -WWW-D -W2-1 --D --1 24-W30-3 -W30-3 --3
YYWWWD 2W3 WWWD W3 D 1 24W3 W303 3

Implied century was possible in ISO 8601:2000, but all truncated formats were removed in the third edition, ISO 8601:2004. For backwards compatibility, however, CCYYMM instead of YYMMDD is invalid. The current edition only accepts formats with reduced accuracy that truncate from the right.

Also, the left-hand truncation used to work slightly different than the first table shows.

Deprecated truncated formats with digits code and alternatives
Format Sample Hyphens Implied field(s) Alternative Wildcards
-YY -2 -24 1 century none XXYY, +XXXYY, –XXXYY, …
--MM --2 --07 2 year -MM XXXXMM, *MM, *-MM, …
---DD ---2 ---24 3 month --DD **DD, *-*-DD, -XX-DD, XXXX-XX-DD, …
-DDD -3 -206 1* year same, DDD *-DDD, *DDD, XXXXDDD, …
-WWW -W2 -W30 1 year same, WWW *-WWW, *WWW, XXXX-WWW, …
-W-D -W-1 -W-3 2×1 week same -WXX-D, *W*D, …
---D ---1 ---3 3 any week --D, D **D, *-*-D, …
-Y-WWW -1-W2 -4-W30 2×1 decade none XXXY-WWW, …
-YWWW -1W2 -4W30 1 decade none XXXYWWW, …
-mm -2 T-37 1 hour :mm T*mm, *:mm, T*:mm, XX:mm, TXX:mm
--ss --2 T--05 2 minute ::ss T**ss, *:*:ss, :*:ss, …

International Standard Calendar/Days of the week


ISO 8601 uses the date the Metre Convention was signed as its reference date, assigning to it the date 1875-05-20 (1875-W21-2) and it also equates 2001-01-01 with 2001-W01-1. Although honorable, an event that can be reconstructed more exactly and independently, e.g. an astronomic one, might be more appropriate, but must result in an equivalent year count and week cycle.

Leap rule[]

The Gregorian leap rule does not spread leap years evenly across the cycle, but this is not a defect of the cycle length itself. Its 400-year cycle results in terminating fractions and it has the benefit, though, that its rule can easily be memorized and calculated, but only for leap days, not for leap weeks. Gregorian leap day rule: Add a day to the second month when the year number is divisible by 4, but when it is divisible by 100 it must also be divisible by 400. The placement of leap weeks follows from that, although it could be defined independently as in Rick McCarty’s Weekdate.

The default leap rule cannot be changed, because the International Standard Calendar is proleptic and formats are backwards-compatible! Therefore, alternate leap rules must be indicated explicitly.

  • The leap cycle is also called an era.
  • A leap cycle absolutely must contain an even number of weeks, i.e. the number of days must be divisible by 7.
  • Both leap rules should be easy to cite and one should be able to determine whether a given year has a leap day or leap week with mental arithmetic.
  • Leap years should be spread as evenly as possible across the leap cycle.
  • The leap cycle should not be too long, say a millennium at worst.
  • A larger leap cycle should approximate the solar year (about 365 days, 5 hours and 49 minutes) better than any shorter cycle. The approximation should be smaller.
    • Otherwise it must have another positive feature to be considered.
  • The leap cycle (or a small integer multiple thereof) should contain an even number of lunations.

There are very few leap ratios that fulfill the basic requirements, the shortest one has 71 leap days and 52 leap weeks in 293 years.

Solar leap cycles with an integer number of 7-day weeks
title years days weeks leap days leap weeks mean days/year mean weeks/year lunations olympiads
Julian 28 10227 1461 7 5 365.25 52.17(857142) 346.325… 7
Dee 231 84371 12053 56 41 365.(24) 52.177489… 2857.071… <58
Qumran 293 107016 15288 71 52 365.242321… 52.177474… 3623.903… >73
Gregorian 400 146097 20871 97 71 365.2425 52.1775 4947.311… 100
Mädler 896 327257 46751 217 159 365.2421875 52.177455… 11081.966… 224
Cycles with an integer number of 4-week months
Dee 924 337484 48212 224 164 365.(24) 52.177489… 11428.285… 231
Qumran 1172 428064 61152 284 204 365.242321… 52.177474… 14495.613… 293

293 and 817-year cycles both provide better approximations than the Gregorian one.

The 293-year cycle curiously has as many leap weeks in a cycle as weeks in a normal year. 31 cycles of 293 years each, i.e. 9083 years, are close enough to 112 341 lunations. A lunation could therefore be defined as 107016 days/cycle * 31 cycles / 112341 months = ca. 29.5305899 days/month. 11 cycles work slightly worse.

Intercalary days[]

  • D = 0 is not the Sunday (7) of the preceding week, but is reserved for use for days outside the week cycle, e.g. in the Fixed Festivity Calendar.
  • DDD = 000 and DD = 00 are likewise intended for a day outside the month, quarter or year cycle.
  • WW = W0 and WWW = W00 are likewise intended for a week outside the month, quarter or year cycle.
  • MM = 00, M = 0, MMM = M00 and MM = M0 are likewise intended for a month outside the quarter or year cycle.

No intercalary item is specified for the standard calendars, though.

The Aristean calendar proposes to use D = 8 for the leap day (-06-31) and the intercalary day (-12-31), but DDD ordinal day of the year would differ by one from normal years for the second half of leap years. This proprietary solution is not (yet) supported.

Date marker[]

work in progress (alpha state), not yet reflected in formal grammar

The new marker ‘D’ may be used in front of any date, like ‘T’ is used before times. It does not carry a meaning of day, but it may be used before dates with implied fields, too, so DD and DDDD are valid and unambiguous, but DDD is neither (see Truncation). That means, D1 = Monday (in the week of the current context), D032 = -02-01 (current year).

Month-based additions and clarifications[]

Proposed month-based formats with digits code
3-month triad
level of detail full date year implied more implied (full) sample
q/y ±CCYY-Q ±4-1 -Q -1 2024‑3
m/q/y ±CCYY-Q-M ±4-1-1 -Q-M -1-1 2024‑3‑1
d/m/q/y ±CCYY-Q-M-DD ±4-1-1-2 -Q-M-DD -1-1-2 --M-DD --1-2 2024‑3‑1‑24
d/q/y ±CCYY-Q-DD ±4-1-2 -Q-DD -1-2 2024‑3‑24
level of detail full date year implied more implied (full) sample
dw/q/y ±CCYY-Q-WW-D ±4-1-2-1 -Q-WW-D -1-2-1 2024‑3‑04‑3
dw/m/y ±CCYY-MM-W-D ±4-2-1-1 -MM-W-D -2-1-1 --W-D --1-1 2024-07‑4‑3

The month year has 365 days in a common year or 366 days in a leap year.

Triad: 3-month quarters[]

Numeric date format
Month of triad
Day of month of triad
Day of triad

Three consecutive months make one of four triads. They are 90 (or 91 with leap day), 91, 92 and 92 days long, respectively, and align with the month year of course. These should not be subdivided into weeks, although that is supported.

The condensed format without hyphens is not supported with these dates, because they would collide with existing ones.

Month year layout
Month ‑01 ‑02 ‑03 ‑04 ‑05 ‑06 ‑07 ‑08 ‑09 ‑10 ‑11 ‑12
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Triad ‑1‑1 ‑1‑2 ‑1‑3 ‑2‑1 ‑2‑2 ‑2‑3 ‑3‑1 ‑3‑2 ‑3‑3 ‑4‑1 ‑4‑2 ‑4‑3
‑1 ‑2 ‑3 ‑4

Week-based additions[]

Proposed week-based formats with digits code
level of detail full date year implied more implied (full) sample
yw ±CCYYW ±4W 2024W
±CCYYQ ±4Q 2024Q
±CCYYM ±4M 2024M
13-week quarts
qw/y ±CCYY-QQ ±4-Q1 -QQ -Q1 2024-Q3
±CCYYQQ ±4Q1 QQ Q1 2024Q3
d/qw/y ±CCYY-QQ-DD ±4-Q1-2 -QQ-DD -Q1-2 -Q-DD -Q-2 2024-Q3-24
±CCYYQQDD ±4Q3 QQDD Q3 invalid 2024Q324
w/qw/y ±CCYY-QQ-WWW ±4-Q1-W2 -QQ-WWW -Q1-W2 -Q-WWW -Q-W2 2024-Q3-W04
d/w/qw/y ±CCYY-QQ-WWW-D ±4-Q1-W2-1 -QQ-WWW-D -Q1-W2-1 -Q-WWW-D -Q-W2-1 2024-Q3-W04-3
m/qw/y ±CCYY-QQ-M ±4-Q1-1 -QQ-M -Q1-1 2024-Q3-1
±CCYYQQM ±4Q2 QQM Q2 2024Q31
d/m/qw/y ±CCYY-QQ-M-DD ±4-Q1-1-2 -QQ-M-DD -Q1-1-2 2024-Q3-1-24
±CCYYQQMDD ±4Q4 QQMDD Q4 2024Q3124
w/m/q/y ±CCYY-QQ-M-WW ±4-Q1-1-W1 -QQ-M-WW -Q1-1-W1 2024-Q3-1-W4
d/w/m/q/y ±CCYY-QQ-M-WW-D ±4-Q1-1-W1-1 -QQ-M-WW-D -Q1-1-W1-1 2024-Q3-1-W4-3
4-week moons
m13/y ±CCYY-MMM ±4-M2 -MMM -M2 2024-M08
±CCYYMMM ±4M2 MMM M2 2024M08
d/m13/y ±CCYY-MMM-DD ±4-M2-2 -MMM-DD -M2-2 -M-DD -M-2 2024-M08-10
±CCYYMMMDD ±4M4 MMMDD M4 invalid 2024M0810
w/m13/y ±CCYY-MMM-WW ±4-M2-W1 -MMM-WW -M2-W1 -M-WW -M-W1 2024-M08-W2
d/w/m13/y ±CCYY-MMM-WW-D ±4-M2-W1-1 -MMM-WW-D -M2-W1-1 -M-WW-D -M-W1-1 2024-M08-W2-3

The week year used herein has exactly 52 weeks (364 days) in a short year or 53 weeks (371 days) in a long year. The term normal year is ambiguous, as it means a short year in the context of week years and a common year in the context of month years.

There is no format which allows to specify the ordinal day of a week year (001 through 364 or 371), although that was possible, e.g. as ±CCYY-DDDD. Three consecutive digits after the marker ‘W’ are already used by the condensed format WWWD.

Week year layout
W01 W02 W03 W04 W05 W06 W07 W08 W09 W10 W11 W12 W13 W14 W15 W16 W17 W18 W19 W20 W21 W22 W23 W24 W25 W26 W27 W28 W29 W30 W31 W32 W33 W34 W35 W36 W37 W38 W39 W40 W41 W42 W43 W44 W45 W46 W47 W48 W49 W50 W51 W52
Q1 Q2 Q3 Q4
Q11 Q12 Q13 Q21 Q22 Q23 Q31 Q32 Q33 Q41 Q42 Q43
M01 M02 M03 M04 M05 M06 M07 M08 M09 M10 M11 M12 M13

Week year

Quart: 13-week quarters[]

Quart layout for day or week of quart
Month Week 1 2 3 4 5 6 7
Mon Tue Wed Thu Fri Sat Sun
1 W01 01 02 03 04 05 06 07
W02 08 09 10 11 12 13 14
W03 15 16 17 18 19 20 21
W04 22 23 24 25 26 27 28
2 W05 29 30 31 32 33 34 35
W06 36 37 38 39 40 41 42
W07 43 44 45 46 47 48 49
W08 50 51 52 53 54 55 56
W09 57 58 59 60 61 62 63
3 W10 64 65 66 67 68 69 70
W11 71 72 73 74 75 76 77
W12 78 79 80 81 82 83 84
W13 85 86 87 88 89 90 91
Quart layout for day or week of month of quart
Month Week 1 2 3 4 5 6 7
Mon Tue Wed Thu Fri Sat Sun
1 W1 01 02 03 04 05 06 07
W2 08 09 10 11 12 13 14
W3 15 16 17 18 19 20 21
W4 22 23 24 25 26 27 28
2 W1 29 30 01 02 03 04 05
W2 06 07 08 09 10 11 12
W3 13 14 15 16 17 18 19
W4 20 21 22 23 24 25 26
W5 27 28 29 30 31 01 02
3 W1 03 04 05 06 07 08 09
W2 10 11 12 13 14 15 16
W3 17 18 19 20 21 22 23
W4 24 25 26 27 28 29 30
Alphanumeric date format
‘Q’ marker
Quart year
Quart of year
Day of quart
Week of quart
Day of Week
Month of quart
Day of month
Week of month
Day of month

Each of the 4 quarters, called quarts, has 13 weeks excatly, except for the final one in long years. This long quart has 14 weeks then.

Although there is no consensus on how quarts of 91 days or 13 weeks should be separated into 3 months of almost equal length, there are just two basic approaches: one divides each quarter into portions of 30 days twice and 31 days once, the other uses 4 weeks twice and 5 weeks once. Choosing the former, the Common-Civil-Calendar-and-Time calendar, the Hanke-Henry Permanent Calendar, the ISO-Uncia Leap Week Calendar and the Edwards perpetual calendar all use 30:30:31 days, the Symmetry010 Calendar uses 30:31:30 days and the Aristean Calendar uses 31:30:30 days. When the “Thursday rule” is applied to any of these patterns it always results in a week layout of 4:5:4 as in the Symmetry454 Calendar, i.e. neither 5:4:4 as in the Bonavian Civil Calendar nor 4:4:5. Months of quarts, furthermore, cannot match exactly the full-week months determined by the week date (-MM-WW or -Q-M-WW), because the first triad may have just 12 weeks and the third triad may also have 14 weeks (like the fourth).

Quarts are therefore divided into three months that primarily consists of 4, 5 and 4 weeks (-QQ-M-WW-D) and, matching that middle-high scheme, alternatively they consist of 30, 31 and 30 days (-QQ-M-DD). Without weeks or days provided, i.e. in the form -QQ-M, there is no distinction between these – the month duality. There is no way to reference a day in 28|35-day months without its week.

‘W’ instead of ‘Q’ as a marker for quarts would work, too, but not as well for some (condensed) formats. Also, it may be counter-intuitive to have “W1” not mean the first week of a month.

Moon: 13 months[]

Moon layout
Week 1 2 3 4 5 6 7
Mon Tue Wed Thu Fri Sat Sun
W1 01 02 03 04 05 06 07
W2 08 09 10 11 12 13 14
W3 15 16 17 18 19 20 21
W4 22 23 24 25 26 27 28
Alphanumeric date format
‘M’ marker
Moon year
Moon of year
Day of moon
Week of moon
Day of week

The week year is divided into 13 months, called moons. A normal moon has 4 complete weeks (28 days). The last moon in long years is a long moon and has 5 weeks (35 days). Since there is a leap week instead of intercalary days, these moons align with the week year, not the month year.

This format is compatible with the New Earth Calendar, which uses a custom leap rule though, and differs from the International Fixed Calendar (Cotsworth–Eastman plan), which uses intercalary days and starts weeks on Sunday.

With alternative leap rules, there can be a 13-moon year with an additional leap moon every 22 or 23 years, but this does not work with a 400-year leap cycle, because it does not contain an integer multiple of 28 days. The 293-year cycle, however, would contain exactly 13 leap moons. Another leap rule may use an independent year count for moon years, of which there are 294 in a cycle of 293 week or month years. There would be no long moons in either case.

International Standard Calendar/Named moons

Mixed additions and clarifications[]

Proposed mixed formats with digits code
level of detail full date year implied more implied (full) sample
w/q/y ±CCYY-Q-WWW ±4-1-W2 -Q-WWW -1-W2 2024-3-W04
d/w/q/y ±CCYY-Q-WWW-D ±4-1-W2-1 -Q-WWW-D -1-W2-1 2024-3-W04-3
w/m/y ±CCYY-MM-WW ±4-2-W1 -MM-WW -2-W1 --WW --W1 2024-07-W2
d/w/m/y ±CCYY-MM-WW-D ±4-2-W1-1 -MM-WW-D -2-W1-1 --WW-D --W1-1 2024-07-W2-3

Week of month or of triad[]

Weeks per month, year and triad depending on the weekday of -01-01; in leap years marked adjacent months switch their week count (“4+ 5–”)
1 Jan: Mon Tue Wed Thu Fri Sat Sun usual
-01 4 5 5 5 4 4 4 57%
-02 4+ 4 4 4 4 4 4 96¾%
-03 5– 4 4 4 4+ 5 5 57%
-04 4 4 4+ 5 5– 4 4 71½%
-05 5 5 5– 4 4 4 4+ 57%
-06 4 4 4 4 4+ 5 5– 67¾%
-07 4 4+ 5 5 5– 4 4 57¼%
-08 5 5– 4 4 4 4+ 5 57¼%
-09 4 4 4 4+ 5 5– 4 71½%
-10 4+ 5 5 5– 4 4 4 57%
-11 5– 4 4 4 4 4+ 5 71½%
-12 4 4 4+ 5 5 5– 4 57%
Year 52 52 52+ 53 52 52 52 82¼%
-1 13 13 13 13 12+ 13 13 89¼%
-2 13 13 13 13 13 13 13 100%
-3 13 13 13 13+ 14– 13 13 86%
-4 13 13 13+ 14– 13 13 13 85½%
Alphanumeric date format
‘W’ marker
Week of triad
Day of week
Week of month
day of week

The number of weeks per month, hence triads, is determined by the usual Thursday rule, that means a week belongs to the month (or triad) the majority of its days (4 to 7) fall into, this always includes its Thursday.

A short month has 4 weeks, a long month has 5 weeks. There are 4 long months in normal years and 5 ones in 53-week long years. The term normal monthis only used for Gregorian months of 28 to 31 days. A month has 5 weeks if it has at least 29 days and starts on Thursday, has at least 30 days and starts on Wednesday, or has 31 days and starts on Tuesday. The resulting pattern is irregular.

The first triad may have just 12 weeks (short triad), the second always has 13 weeks (normal triad) and either the third or the fourth may, instead of 13, have 14 weeks (long triad).

Note that triads and normal months divided into full weeks together effectively constitute the week year and not the month year. To put it differently: every date with a ‘W’ marker in it uses the week year.


Existing formats[]

Existing formats with digits code
level of detail full date
h hh 2
m/h hh:mm 2:2
hhmm 4
s/m/h hh:mm:ss 2:2:2
hhmmss 6
f .f .1 … .9
,f ,1 … ,9
f/h hh.f 2.1 … 2.9
not applicable
f/m/h hh:mm.f 2:2.1 … 2:2.9
hhmm.f 4.1 … 4.9
f/s/m/h hh:mm:ss.f 2:2:2.1 … 2:2:2.9
hhmmss.f 6.1 … 6.9

Decimal time[]

Decimal time of day is valid without ‘T’ prefix: “.5” and “,5” mean 12:00:00. The decimal part (with comma) may also start after year, quarter (i.e. quart or triad), month (incl. moon), week and, of course, day. No further subdivisions are possible then.

Time zone[]

Time zone formats with digits code
level of detail positive offset negative offset
h +hh +2 hh -2
not applicable
m/h +hh:mm +2:2 hh:mm -2:2
+hhmm +4 hhmm -4

The negative zero time zone offset -00:00 or -00 is valid and, as in RFC 3339, it explicitly specifies that there is no preferred timezone.

Intervals, spans, periods, repetitions[]

under construction
  • ‘Q’ is added for the 13|14-week quart, 3-month triads remain “3M”
  • A duration may now combine weeks with years and days, but not with months. ISO 8601:2004 only allowed PnW, but not PmYnWoD. A year, in this case, consist of either 52 or 53 complete weeks.
  • When a time interval is specified by start and end date, both should be provided in the same format.

Templates and comments[]

under construction


Comments are placed between an opening angular bracket ‘<’ and a closing one ‘>’. A conforming software implementation may ignore anything after the comment start character and the matching comment end character or the end of the date string. This may be used, among other things, to tag a month-day date with its weekday, e.g. 2012-09-10<Monday>.


With a calendar reform there are always several ways to determine the date of annual holidays and birthdays.

  • Convert from the classic calendar each year, e.g. Christmas, December 25, stays at -12-25 and can fall on any day of the week.
    • A special case are astronomically defined holidays which use features that are not accurately represented in the calendar, e.g. four days after the winter solstice (could be written -S0-1-04 or -S4-04 etc.). They have to be determined by observation or, rather, by independent calculation.
  • Convert the original date to the new calendar, e.g. 0000-12-25 was a Monday, hence -W52-1.
  • Use a similar looking date in the new calendar, e.g. -M12-25 which is a Thursday and equals -M12-W4-4.
  • Reinterpret the date in the new calendar, e.g. three weeks and four days into the last month of the year, -12-W3-4 (Thursday), or one week before the last day of the year, -W51-7 (Sunday).

Depending on the reason for a holiday one or several of these methods may make sense to use. Note that stakeholders may prefer different approaches for external reasons, workers may prefer to have holidays not fall on weekends, for example.

Week days of some popular holidays[1]
Dom. Frequency *-01-01 *-01-06 *-02-14 *-05-01 *-11-01 *-12-25
C 43 ⇒ 10¾% W53-5 W01-3 W06-7 W17-6 W44-1 W51-6
CB 15 ⇒ 3¾% W17-7 W44-2 W51-7
B* 13 ⇒ 3¼% W53-6 W01-4 W07-1
B 30 ⇒ 7½% W52-6
BA 13 ⇒ 3¼% W18-1 W44-3 W52-1
A 43 ⇒ 10¾% W52-7 W01-5 W07-2
AG 15 ⇒ 3¾% W18-2 W44-4 W52-2
G 43 ⇒ 10¾% W01-1 W01-6 W07-3
GF 13 ⇒ 3¼% W18-3 W44-5 W52-3
F 44 ⇒ 11% W01-2 W01-7 W07-4
FE 14 ⇒ 3½% W18-4 W44-6 W52-4
E 43 ⇒ 10¾% W01-3 W02-1 W07-5
ED 14 ⇒ 3½% W18-5 W44-7 W52-5
D 44 ⇒ 11% W01-4 W02-2 W07-6
DC 13 ⇒ 3¼% W18-6 W45-1 W52-6
Week days of some national holidays, dow of original date highlighted
Dom. Frequency US France Germany
1776-07-04 1789-07-14 1990-10-03
C 43 ⇒ 10¾% W26-7 W28-3 W39-7
CB 15 ⇒ 3¾% W27-1 W28-4 W40-1
B* 13 ⇒ 3¼%
B 30 ⇒ 7½%
BA 13 ⇒ 3¼% W27-2 W28-5 W40-2
A 43 ⇒ 10¾%
AG 15 ⇒ 3¾% W27-3 W28-6 W40-3
G 43 ⇒ 10¾%
GF 13 ⇒ 3¼% W27-4 W28-7 W40-4
F 44 ⇒ 11%
FE 14 ⇒ 3½% W27-5 W29-1 W40-5
E 43 ⇒ 10¾%
ED 14 ⇒ 3½% W27-6 W29-2 W40-6
D 44 ⇒ 11%
DC 13 ⇒ 3¼% W27-7 W29-3 W40-7

Astronomically defined holidays can of course be fixed arbitrarily in any calendar. The date of Easter in non-orthodox churches, for instance, is currently specified as the first Sunday after the first full moon after the begin of spring in the Gregorian calendar (-03-21). One could instead use the corresponding week from the year Jesus of Nazareth supposedly died on the cross, or one selects the day that is most frequently selected by the current rule or is closest to the median, -W14-7.


under construction

A fully compliant software implementation of this specification must be able to accept any of the formats described and convert it into every other representation. A partially compliant implementation must accept at least one of the formats and must not successfully parse any otherwise valid format into something else.