ISO 8601-1:2019,Date and time — Representations for information interchange — Part 1: Basic rules.
You can download ISO 8601-1:219 PDF from this website for free. We should thank someone called Mary, who I think Shared the document with me via email a few days ago.
Here are some of my selections. You can read more about this document after you download it yourself.
ISO 8601-1:219 specifies representations of dates of the Gregorian calendar and times based on the 24-hour clock, as well as composite elements of them, as character strings for use in information interchange. It is also applicable for representing times and time shifts based on Coordinated Universa Time (UTC).
The Gregorian calendar provides a time scale consisting of a series of contiguous calendar years, each identified by a year number represented by an integer, greater than that of the immediately preceding calendar year by 1. This document allows the identification of calendar years by their year number for years both before and after the introduction of the Gregorian calendar.
The Gregorian calendar distinguishes common years of 365 consecutive calendar days and leap years of 366 consecutive calendar days.
If you feel that this standard is your urgent need, then you can download it for free.As the webmaster of this site,I  sincerely hope to help you. If you think this website is nice, you can help me publicize it. Of course, you can also leave a message below.
If you need ISO 8601-2:2019, you can click the text link on the left for more information.
Download link:ISO 8601-1:2019 pdf