EDTF

The Extended Date/Time Format (EDTF) was primarily specified by the US Library of Congress (LoC) and became part of ISO 8601-2:2019. It has three levels, 0–2. Each level supports all features of all lower levels. Some of its extensions to ISO 8601-1 or their scopes are similar to what the International Calendar provides over the International Standard Calendar, but they differ in the details.

Level 0[]

The base Level 0 has only features from ISO 8601-1:2019 (i.e. Part 1), which in turn is basically equivalent to the previous complete edition ISO 8601:2004. Anything that is in Part 1 but not in Level 0, must be ignored during EDTF-conformant parsing. In particular, EDTF requires extended format with hyphen - separator in date parts and colon : separator in time parts, not basic format without those.

Level 0 only supports dates in complete CCYY-MM-DD and reduced precisions, CCYY-MM and CCYY, i.e. no ordinal dates CCYY-DDD and no week dates CCYY-WWW-D, CCYY-WWW.

Times are only supported as part of a datetime, i.e. the T separator cannot be omitted and the part before it must not be empty. All times must be complete to the second and do not support fractional parts. Timeshifts (vulgo time zones or offsets) are supported, including Z.

Time intervals are only supported with start and end date (separated by a forward slash /), no durations and no datetimes. Repetitions are also not part of EDTF, Level 0 or otherwise.

Extensions of Part 1[]

Years[]

EDTF supports negative years without further agreement between communication partners required, as it is in Part 1. As usual, the Hyphen-Minus character U+002D is supported as a minus sign, but it is ambigious whether the actual Minus character − U+2022 should also be supported.

Level 1 introduces a letter-prefixed calendar year format with the prefix Y. Months and days are not used with this format and the length of the year field must exceed four digits. It is unclear wheter a plus sign + is valid between the prefix and the positive year number. Early drafts had put the sign before the prefix.

Level 2 extends this format further for exponential years, wherein a (possibly signed) integer after the Y prefix may be followed by an E infix and a positive integer for the exponent, i.e. the power of ten to be applied.

Earlier drafts used lowercase letters y and e instead.

The International Calendar, in comparison, simply relies on a mandatory sign for years (also within dates) with five to ten digits.

Year subdivision[]

EDTF overloads the month field MM with arbitrary numbers above 20.

Level 1 only supports the first four values for seasons that are not further specified:

21: Spring
22: Summer
23: Autumn, Fall
24: Winter

This is only valid in reduced precision dates CCYY-MM, i.e. the day field DD must not be used with these. They are valid in time spans. It is left undefined when these seasons would start or end. In particular, it is left ambiguous which hemisphere they use, whether they consist of full months or astronomic seasons, and whether (Northern) winter or (Southern) summer comes at the start or the end of the year. These formats were originally intended for libraries that need to catalogue quarterly issues of magazines and the like.

Level 2 adds more arbitrary numbers as sub-year groupings for MM:

25: Northern Spring
26: Northern Summer
27: Northern Autumn
28: Northern Winter
29: Southern Spring
30: Southern Summer
31: Southern Autumn
32: Southern Winter
33: Quarter 1 (January, February, March)
34: Quarter 2 (April, May, June)
35: Quarter 3 (July, August, September)
36: Quarter 4 (October, November, December)
37: Quadrimester 1 (January, February, March, April)
38: Quadrimester 2 (May, June, July, August)
39: Quadrimester 3 (September, October, November, December)
40: Semestral 1 (January–June)
41: Semestral 2 (July–December)

The differentiation between 3-month quarters , i.e. calendar trimesters, and seasons implies that the latter are supposed to use astronomic definitions, i.e. running from equinox through solstice or vice versa. Some seasons are sematically different but chronologically equal: 25 = 31, 27 = 29 and probably 26 = 32 and 28 = 30.

Data quality[]

EDTF adds single-character qualifiers.

question mark ?: uncertain (possible, but not definite, e.g. partially readable source)
tilde ~: approximate (estimated)
percent sign %: both, uncertain and approximate

There can only be one qualifier in each position, earlier drafts did not have % and therefore allowed to combine ? and ~.

Level 1 supports these for complete and reduced-precision dates as suffixes.

Level 2 extends them to individual date fields (or components) as prefixes and allows suffixes after non-terminal fields to indicate the quality of this and all higher fields (group qualification).

Earlier drafts only had suffixed qualifiers and instead supported parentheses () for group qualification.

EDTF officially does not seem to support qualifiers for times and time fields in datetimes. These are also valid for the start date or end date or both in interval notation with the usual restrictions for the level.

Precision[]

EDTF introduces a wildcard placeholder character X for unspecified digits from the right. Earlier drafts used the letter u instead. In Level 1, this can only be applied to one or two year digits Y but not to the century digits in a year number CCYY. It is unclear whether it needs to be supported in longer, letter-prefixed calendar years. In Level 1, day DD and month MM fields can only be unspecified numbers, i.e. both theit digits may be XXy or none, not one only. In Level 2, unspecified digits X may occur anywhere within any component. EDTF is not clearly saying whether they are also valid in the time part of datetimes, but is explicit about them being valid in intervals.

In extended intervals (vulgo time spans), EDTF supports the omission of either the start or the end date when it is unknown (null string). If either one shall be left out for other reasons, it can be replaced by a double dot .. placeholder. Earlier drafts used literal English keywords unknown and open for tese purposes, respectively.

EDTF Level 2 introduces significant digits for year numbers in all supported formats (±CCYY, Y±ECCYY, YEEN) which is a positive (usually single-digit) integer prefixed by S coming after the whole year field (i.e. after an exponent). In practice, one gives an estimated year number and the number of digits from the left that are certain. Earlier drafts used the marker P instead, for precision, and had something called masked precision.

Sets[]

EDTF Level 2 introduces set representations with square brackets [] and curly braces {}. They must always occur in pairs. Exclusive or single-choice lists, of which only one member applies, are put inside square brackets. and inclusive lists, where all members apply, are enclosed by curly braces. In both cases, members are separated by a comma without any whitespace being allowed. To indicate inclusive ranges of members, double dots .. may be used, and if start or end is left out it becomes a one-sided open range. Ranges must be noted from earlier to later, but the order of list items is not important otherwise.

Proposals for Level 3[]

There is no Level 3 in the 2019 edition of the standard, but several further extensions seem useful and viable.

Level 3 should allow placeholder X in the time part of datetimes and in standalone times, but not within timezone offsets. This would be similar to wildcards in the International Calendar.

Level 3 should introduce additional sub-year groupings to unambiguously refer to all possible 3-, 4- and 6-month groups, but the actual numbers are open to discussion:

3-month trimesters (or quarters)
- January, February, March (arguably 33, 1 in IC)
- April, May, June (arguably 34, 2 in IC)
- July, August, September (arguably 35, 3 in IC)
- October, November, December (arguably 36, 4 in IC)
- February, March, April
- May, June, July
- August, September, October
- November, December, January
- March, April, May
- June, July, August
- September, October, November
- December, January, February
4-month quadrimesters (or quadmesters or tertials)
- January, February, March, April (arguably 37)
- May, June, July, August (arguably 38)
- September, October, November, December (arguably 39)
- February, March, April, May
- June, July, August, September
- October, November, December, January
- March, April, May, June
- July, August, September, October
- November, December, January, February
- April, May, June, July
- August, September, October, November
- December, January, February, March
6-month semestrals (or semesters or half-years)
- January, February, March, April, May, June (arguably 40)
- July, August, September, October, November, December (arguably 41)
- February, March, April, May, June, July
- August, September, October, November, December, January
- March, April, May, June, July, August
- September, October, November, December, January, February
- April, May, June, July, August, September
- October, November, December, January, February, March
- May, June, July, August, September, October
- November, December, January, February, March, April
- June, July, August, September, October, November
- December, January, February, March, April, May

If EDTF was extended to cover week dates, 13-week quarters would also be a logical addition (Q1 through Q4 in IC).

Level 3 could also introduce the month-like spans of astronomic zodiac signs, of which three make an astronomic season. They start, by tropical definitions, between the 19th and 23rd day of a standard month (provided in parentheses). In IC, these can be referred to as either S01 through S12 or, with season, from S1-1 to S4-3.

♑︎︎ Capricorn (22 December)
♒︎︎ Aquarius (20 January)
♓︎︎ Pisces (19 February)
♈︎︎ Aries (21 March)
♉︎︎ Taurus (20 April)
♊︎︎ Gemini (21 May)
♋︎︎ Cancer (20 June)
♌︎︎ Leo (23 July)
♍︎︎ Virgo (23 August)
♎︎︎ Libra (23 September)
♏︎︎ Scorpio (23 October)
♐︎︎ Sagittarius (22 November)