Clarify which revision of ISO 8601 Chrono adheres to

[xTibor]

The README states Chrono strictly adheres to ISO 8601. I think this statement is slightly ambiguous, there are multiple revisions of the standard with minor differences. Would be nice to know which one Chrono actually follows. (2004?, 2019?)

[pitdicker]

I don't think chrono adheres all that strictly, more that it drew a lot of inspiration from ISO 8601. About the revisions ISO 8601:2019 contains this note (https://www.iso.org/obp/ui/en/#iso:std:iso:8601:-1:ed-1:v1:en):

The main changes compared to ISO 8601:2004 are as follows:

• conversion of the content as Part 1 with the Part title “Basic rules” due to the addition of another Part 2 “Extensions” of ISO 8601;
• replacement of the term “midnight” with “beginning of day”, disallowing the value “24” for hour;
• update of terms and definitions:
• “time point” is now “time”;
• “local time” is now “local time of day”;
• added definition for “time of day” and “local time scale”;
• updated definitions for “standard time of day”, “local time of day” and “UTC of day” to rely on “time of day”;
• combined two “day” terms in different domains for consistency;
• change of the representation of “leap seconds”;
• clarification of “calendar day” expressions intended to mean “calendar day of week” (etc.);
• amendment of the recurring time interval (3.1.1.11) to provide a link to ISO 8601-2:2019 which
  contains in Clause 5 the “repeat rules for recurring time intervals”.

So mostly changes to terminology.

"disallowing the value “24” for hour;" is interesting. Chrono never supported that. Yet for an ISO 8601 parser in #1143 I think it makes sense to support both the newer and older standard. I.e. to support the value 24 for hours to main midnight at the end of the day.

[xTibor]

I don't think chrono adheres all that strictly, more that it drew a lot of inspiration from ISO 8601

Just for reference, strict ISO 8601 adherence was the first feature chrono stated in its README file when I filed this issue. It has been removed from the README since then.

chrono/README.md

Line 22 in 8bd13db

* Chrono strictly adheres to ISO 8601.

[pitdicker]

I understand. That claim has been there since pretty much the first release of chrono. At the time it was written the revision was 2014.

But you opened a good issue. If we promise to strictly follow a standard, it should also correctly reference the standard including revision.

For my work every rapport and all the software starts with a list of the relevant building codes, which have to be specified including revision, amendments, corrections and national annexes.

I just don't know yet how much chrono should advertise the ISO 8601 support in the main documentation. Maybe it should not be more than a section that documents the relevant parsing and formatting methods, as ISO 8601 is mostly for interchange and does not specify how to do calculations on date and time values.

[pitdicker]

@xTibor Do you have experience with this? Does it sometimes come up in a list of requirements like "the software must support ISO 8601:2019", and libraries must be picked to match? I hope you understand what I mean to say...