ISO 8601:2019

ISO 8601:2004 has been superseded by ISO 8601:2019 which now contains two parts. Part 1 specifies basic rules. Part 2 specifies extensions, including community profiles to specify conformance levels for use of which rules.

Part 1 Basic Rules

The main changes of interest are representing positive leap seconds as [23:59:60Z] and disallowing the value “24” for hour, so that there is no “end of day” overlap with the beginning of the day [00:00.00Z].

The extended format date times have separators (YYYY-MM-DDThh:mm:ss.i). The basic format date times are without separators, except that basic times always start with “T”. Year is ordinarily an ordinal between 0000 and 9999, however expanded representations allow larger and negative years by mutual agreement. A time may include a decimal fraction (seperated using comma or period) in the lowest order time scale component.

A complete representation of duration can be expressed as a combination of units with designators starting with “P” : PYYMMDDTHHMMSS . By mutual agreement the basic and extended formats prefixed with “P” can be used.
The application of duration to dates is described in Part 2 Extensions.

Part 2 Extensions

Many extensions are provided that may be of interest. For examples:

explicit representation of date time, with designators
as for duration without the “P”.
year before year one. eg 1YB = 0Y. (1 B.C.)
negative and exponential date and time components, in explicit form.
Grouped Units of time scales. eg. 2G14DU = 2nd fortnight.
Sets of date times, and their expansions.

Evaluation of duration and date time

An informative annex describes several methods for evaluation.
Composite duration method directly applies the duration’s units to the date and time without truncation, and to apply carry over at the end.

(Note that this contradicts the XSD specification of applying months first, and then seconds wiht carry over.)

iCalendar

ISO 8601-2:2019 uses iCal (RFC 5545) as a basis for specifying recurrence, as
RNumberOfOccurrences followed by one of

/Duration in context
/Start/End
/Start/Duration
/Duration/End

optionally followed by a repeat rule, as name value rule pairs.

USA Library of Congress
Extended Date Time Format

“EDTF functionality has now been integrated into ISO 8601-2019”
The USA’s Library of Congress specifies an Extended Date Time Format, incuded as profile in an annex.

It adds indicator characters as suffixes to a date (level 1),
or to portions of a date (level 2) as a suffix to affected portions or as a prefix to a portion:

? = uncertain
~ = approximate
% = uncertain and approximate

It adds a replacement character [X] meaning unspecified, which can be used from the least precision (level 1) or anywhere in the string (level 2).

For level 2, It adds the ablity to express years with exponents, and to specify number of significant digits; divisions of a year into seasons, quarters, semestrals and quadrimesters; and the ability to specify multiple dates with {braces}, one of a set with [square brackers], or a time interval with an unknown (blank) or open (double dot ..) start or end.

Week calendar

ISO 8601 defines a week numbering calendar in which the first calendar week contains the first Thursday of the year, and whose weeks start with Monday as day number 1. The basic format is yyyyWwwd and the extended format is yyyy-Www–d. e.g. 2023-02-05 is also 2023-W05-7.
Further information is available at: ISO week date – Wikipedia and The Mathematics of the ISO 8601 Calendar – Long and Short Years (uu.nl) which explains the pattern of 53 week long years across the 400 year Gregorian calendar cycle.