[asabil]

ISO8601 is a standard managed by ISO, you can buy a digital or printed copy of the standard from here: https://www.iso.org/standard/70907.html

RFC 3339 is managed by the IETF, and their documents are accessible free of charge.

[mananaysiempre]

> you can buy a digital or printed copy of the standard from [the ISO]

Or from your national member body, which for some standards and countries can be substantially cheaper.

[seabass-labrax]

In Britain, standards can sometimes be obtained for free via your library subscription (which would be orders of magnitude cheaper than the standards are directly, due to government subsidies for libraries).