The date formats like 6321 or 6321 are not used because the lead to dash in some countries
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Answer:
The legal and cultural expectations for date and time representation vary between countries, and it is important to be aware of the forms of all-numeric calendar dates used in a particular country to know what date is intended.
Writers have traditionally written abbreviated dates according to their local custom, creating all-numeric equivalents to day–month formats such as "18 August 2021" (18/08/21, 18/08/2021, 18-08-2021 or 18.08.2021) and month–day formats such as "August 18, 2021" (08/18/21 or 08/18/2021). This can result in dates that are impossible to understand correctly without knowing the writer's origin and/or other contextual details, as dates such as "10/11/06" can be interpreted as "10 November 2006" in the DMY format, "October 11, 2006" in MDY, and "2010 November 6" in YMD.
The ISO 8601 format YYYY-MM-DD (2021-08-18) is intended to harmonize these formats and ensure accuracy in all situations. Many countries have adopted it as their sole official date format, though even in these areas writers may adopt abbreviated formats that are no longer recommended.