Proposal to switch from ISO 8601 to IETF RFC 3339

[nathansalter]

Many specifications and software describe their date usages as ISO 8601. However, there are many places where the software doesn't fully adhere to the specification [1][2]. This is partly because the people implementing the standard may have not had full access to the specification. The issue is that ISO 8601 is covered by copyright, and requires a payment to be able to download the PDF [3]. The IETF RFC standard was specifically designed for internet headers, so is a more suitable format for the APIs and models.

[1] https://www.php.net/manual/en/class.datetimeinterface.php#datetime.constants.iso8601
[2] https://pypi.org/project/iso8601/#where-it-differs-from-iso-8601
[3] https://www.iso.org/standard/70907.html