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The word palindrome is Greek — palin, again, and dromos, course — but it is not an ancient word: English usage is generally credited to the writer Henry Peacham in 1638. Palindromic dates are younger still, because they are not really facts about time. They are facts about notation, and the notations are recent.

Which days qualify depends entirely on the order the fields are written in, and that order is a regional accident. Most of the world writes day-month-year; the United States writes month-day-year; ISO 8601, first published by the International Organization for Standardization in 1988, prescribes year-month-day. The ISO order is the one used in filenames, log lines and database columns because it is the only one of the three that sorts chronologically as plain text — which also means its palindromes are a very modern category.

The modern enthusiasm dates from the 2010s, when two-digit years produced runs of palindromes in the American M/D/YY order — "palindrome weeks" such as 5/10/15 through 5/19/15 — and spread on social media. Aziz S. Inan, a professor of electrical engineering at the University of Portland in Oregon, has spent years cataloguing date palindromes and related numerical patterns, and is the person journalists tend to call when one comes round.

Two days drew the most attention. 2 February 2020 is genuinely unusual: 02/02/2020 reads the same in all three orders at once, which had not happened since 11/11/1111 and will not happen again until 12/12/2121. The 2021 case is subtler and often misreported. The digit string 12022021 is a palindrome and a valid date under both conventions — 12 February in Britain, 2 December in America — so the written form is "universal" even though the day is not: 2 December 2021 written the British way, 02/12/2021, is not a palindrome at all. This page computes each format separately, so the difference is visible rather than glossed over.

主な性質

• A date is a palindrome only relative to a written format. 2 December 2021 is 12/02/2021 in MM/DD/YYYY, which mirrors, and 02/12/2021 in DD/MM/YYYY, which does not.
• In the eight-digit DD/MM/YYYY form the year is forced: the mirror condition makes YYYY exactly DDMM written backwards, so each day-and-month pair has at most one palindromic year.
• In YYYY-MM-DD the month and day are forced instead — MM is the last two digits of the year reversed, DD the first two reversed — so a year contains at most one ISO palindrome.
• There are exactly twelve YYYY-MM-DD palindromes between the years 2001 and 2100, and every one of them falls on the 2nd of its month.
• 02/02/2020 mirrors in DD/MM/YYYY, MM/DD/YYYY and YYYY-MM-DD simultaneously; the previous such day was 11/11/1111 and the next is 12/12/2121.
• A mirrored digit string is not automatically a date: 30/02/2003 and 31/04/4013 both mirror correctly, but February has no 30th and April no 31st, so both are rejected here.
• Leap years matter. 29/02/2092 is a valid DD/MM/YYYY palindrome because 2092 is divisible by 4 and is not a century year.
• The leap rule applied is the Gregorian one: divisible by 4, except century years, unless those are divisible by 400.

登場する場面

• 2 February 2020 was reported worldwide as a "universal palindrome day", the first in 909 years — the previous one having been 11/11/1111.
• Aziz S. Inan of the University of Portland publishes on date palindromes and numerical date patterns and is widely quoted in press coverage of them.
• Registrars and wedding venues have reported booking spikes on palindromic and repeated-digit dates such as 11/11/11 and 02/02/2020. The idea that such dates are auspicious is a belief, not a documented effect.
• ISO 8601's YYYY-MM-DD is standard in filenames, log timestamps and database columns because it sorts chronologically as text; its palindromes are a by-product of a format designed for machines.
• "Palindrome weeks" circulated widely on social media through the 2010s: runs of consecutive days that mirror in the American M/D/YY order, such as 5/10/15 to 5/19/15.