ECMAScript specifies "simplified extended ISO 8601", which is just that string above.
That also reminds me that .NET documentation likes to pedantically remind me that what I think of as ISO 8601 is probably more specifically IETF RFC 3339, which defines a formal BNF grammar as ISO didn't think to do that in the 80s. (Yay, standards.)
It took me quite a while staring at Stack Overflow answers and my code, wondering why the SO answers supposedly worked and my code did not, before I realized I had time.strptime and they were using datetime.strptime.
python-dateutil for incomplete but stdlibbish ISO 8601:2004.
python-edtf for ISO 8601-2:2019.
However, the ISO 8601-2:2019 spec is something of a clusterfuck born out of EDTF.
Most public discourse about it can be found in various confused crossover threads from people who need open-ended dates for their projects.[2-9]
Here's a haskell implementation of ISO 8601-1:2019 and ISO 8601-2:2019, for reference.[10]
[1]: https://github.com/metomi/isodatetime/issues/138
[2]: https://www.loc.gov/standards/datetime/
[3]: https://www.loc.gov/standards/datetime/background.html
[4]: https://www.loc.gov/standards/datetime/implementations.html
[5]: https://github.com/plk/biblatex/issues/656
[6]: https://github.com/ixc/python-edtf/issues/24
[7]: https://github.com/saw-leipzig/csv2cmi/issues/18
[8]: https://github.com/schemaorg/schemaorg/issues/242
[9]: https://github.com/JohnLukeBentley/open-datetime-standard-bo...