Context: Japanese dates count years from the accession of the current emperor (Reiwa 1 = 2019, Reiwa 2 = 2020, etc), so when the emperor changes, you need to renumber basically everything.
Bonus: you also get confused foreigners who think "6/12/20" is 6 Dec 2020 or even 12 June 2020, instead of 20 Dec 2024 (Reiwa 6). Fortunately the Japanese themselves are ISO-compliant and always use YYMMDD.
When/if an emperor reigns for over 100 years, would they shorten the format as-per-iso shorthand rules? That is, would the year Reiwa 106 also be written as 6/12/20 ?
Now let's just hope Australia follows suit...by eliminating the crown entirely from government.