Global Palindrome Day

By Susam Pal on 02 Feb 2020

Date Formats

There are three popular date formats followed worldwide: big-endian, little-endian, and middle-endian. Here is how today's date looks like in these formats if the year is written out in full:

Endianness Format Date
Big-endian YYYY-MM-DD 2020-02-02
Little-endian DD-MM-YYYY 02-02-2020
Middle-endian MM-DD-YYYY 02-02-2020

The ISO 8601 standard specifies the big-endian date format. The big-endian format is popular in Canada, China, Japan, Lithuania, etc. The little-endian format is used by the majority of the world including countries like Brazil, Egypt, Germany, India, Italy, United Kingdom, etc. The middle-endian date format is primarily used in the Philippines and the United States.

The preferred choice of delimiter between the date components (day, month, and year) varies from country to country. In this post, we use hyphen (i.e., "-") as the delimiter.

Palindromes

Today's date is a palindrome in all three date formats, i.e., it reads the same backward as forward in all three formats. It is globally palindromic. Note that we ignore the hyphens or any other delimiters while deciding if a date is palindrome or not. In fact, in general, punctuation, capitalization, and spaces are usually ignored while deciding if a given text is palindrome or not.

Let me define global palindrome day to be a day when its date is a palindrome in all three date formats (YYYY-MM-DD, DD-MM-YYYY, and MM-DD-YYYY). Then today is a global palindrome day!

There are only eleven such global palindrome days between the years 1000 and 9999. Here is a list that enumerates all of them:

1010-01-01
1111-11-11
2020-02-02
2121-12-12
3030-03-03
4040-04-04
5050-05-05
6060-06-06
7070-07-07
8080-08-08
9090-09-09

The previous global palindrome day was over 908 years ago! The next such day is over 101 years away!

Today is special in another way. It is day 33 (a palindrome) of the current year and 333 (also a palindrome) days remain in the year.

Ambigrams

As a bonus, today is also a global strobogrammatic day when viewed on a seven-segment display, i.e., for all three date formats, today's date when shown, without the delimiters, on a seven-segment display appears the same when rotated by 180°. A strobogrammatic sequence of digits like this is also a type of ambigram. Here is how today's date looks in YYYYMMDD, DDMMYYYY, and MMDDYYYY formats on a seven-segment display:

Date 2020-20-20 (YYYY-MM-DD) shown in a seven-segment display
Date 02-02-2020 (DD-MM-YYYY) shown in a seven-segment display
Date 02-02-2020 (MM-DD-YYYY) shown in a seven-segment display

In the list of global palindrome days provided in the previous section, six of the eleven days are global strobogrammatic days too. Only the ones that belong to the years 3030, 4040, 6060, 7070, and 9090 are not strobogrammatic on a seven-segment display; the rest all are.