changed enitre logic for isIso8601 and it's test cases

[jesroffrouk]

feat(isISO8601): Rewrote entire logic for isISO8601 to solve the issue of #2003

I rewrote the entire logic for isISO8601 to fix several unhandled edge cases and improve overall reliability. Specifically, this addresses:

Week-based date formats (e.g. 2023-W52-7)

Milliseconds without seconds (e.g. 2023-03-15T13:30.123Z)

Incorrect acceptance of dates like 2016-W53 due to regex-only checks

This also resolves #2003.

Key changes:

Removed strict and strictSeparator options, as they don’t align with ISO 8601:2019, which mandates the 'T' separator.

Consolidated validation into a single regex, followed by actual date component checks (rather than relying on regex to determine validity).

Added comprehensive handling for leap years, week formats, time zones, and date boundaries.

Skipped support for astronomical years and basic format (e.g., 20251231) — these can be added later if needed, but are uncommon in modern usage.

The updated validator now handles all extended ISO 8601 formats I could find and passes against a wide range of edge cases.

References:

• ISO 8601:2019 standard overview: https://www.iso.org/iso-8601-date-and-time-format.html
• Wikipedia overview of ISO 8601: https://en.wikipedia.org/wiki/ISO_8601
• GitHub issue being fixed: isISO8601 returns true for an invalid iso8601 date #2003

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fdc788) to head (36a3184).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2564   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files          114       114           
  Lines         2587      2603   +16     
  Branches       656       659    +3     
=========================================
+ Hits          2587      2603   +16     

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[jesroffrouk]

@rubiin can you please review this PR ?

[WikiRik]

One quick comment to resolve the warning from codecov.

Also; this is a breaking change and it might be a while before we merge this. #2398 is also likely to be part of the next major release
@profnandaa is in charge of deciding when we want to go ahead with a new major release

[jesroffrouk]

Thanks for the heads-up! I’ll look forward to the Codecov warning soon.
Totally understand that this is a breaking change—I’m happy to provide support for anything related to this change in the next major release.
I’ll keep an eye out for updates from @profnandaa on that.

[jesroffrouk]

@profnandaa Could you please share an approximate timeline for the next major release? If it's going to take some time, I’ll go ahead and fix the current issue #2003 with minimal tweaks. Otherwise, I can wait and align with the upcoming release.

[rubiin]

fix mcs

[jesroffrouk]

working on it asap

[jesroffrouk]

Docs: kept my own changes for isISO8601 and aligned the remaining sections with the latest master branch. Merge conflicts were resolved only in the README file