From the article: > Example 1 '156X-12-25' > December 25 sometime during the 1560s Such a historical use case comes with a huge caveat. ISO-8601 i

From the article: > Example 1 '156X-12-25' > December 25 sometime during the 1560s Such a historical use case comes with a huge caveat. ISO-8601 is based on the proleptic Gregorian calendar (which extends the Gregorian calendar backwards in time before its introdution), and it also includes a year zero (which is not necessarily included in every proleptic Gregorian calendar). If we use such proleptic dates, we need to take into account that they differ from the Julian calender dates typically used for this time period. '156X-12-25' does not denote the first day of Christmas in the 1560s, but 10 days before Christmas in the 1560s. '156X-XX-XX' describes the interval from 22 Dec 1559 to 21 Dec 1569. And to denote the day of Caesar's death, 15 March 44 BC, we have to write: '-0043-03-13'.