12 ms·
The nice thing about today's date, 2020-02-02, is that it is palindrome in three date formats: YYYYMMDD, DDMMYYYY, and MMDDYYYY. This should probably be called
by susam 7y ago
The nice thing about today's date, 2020-02-02, is that it is palindrome in three date formats: YYYYMMDD, DDMMYYYY, and MMDDYYYY.
This should probably be called "Universal Palindrome Day" because it is a palindrome irrespective of the endianness (big, little, or middle) that is used in various countries.
Here is the list of all such dates until the year 9999:
0101-10-10
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 next such one is over 101 years away from now.
Here is my own blog post on this topic: https://susam.in/blog/universal-palindrome-day/ https://susam.in/blog/universal-palindrome-day/ :-)
- ph2082 7y agoSo it happened like 900 yrs ago and after next one it almost take 900 yrs for same date to happen. And I am alive on one of this rare event. Need to cut Cake. :)
- felipemnoa 7y agoNot to sound corny but if you really think about it every single day is rare, just in different ways.
- susam 7y agoReminds me of the interesting number paradox[1]. This can be easily extended to dates too in this manner: Every date is interesting. Proof by contradiction: If there exists a non-empty set of uninteresting dates, there would be an earliest date in this set. But the earliest such date is itself interesting because it is the earliest uninteresting date, thus producing a contradiction. [1]: https://en.wikipedia.org/wiki/Interesting_number_paradox https://en.wikipedia.org/wiki/Interesting_number_paradox
- mmhsieh 7y agothere has got to be a better objective method to define interesting numbers other than inclusion in a number sequence compendium. perhaps if we hypothesize that there are a (countably) infinite number of number theoretic theorems, might every number be either a unique case or the largest or the smallest example of some instance?
- Retric 7y agoThere is no largest or smallest real number. Therefore a set without a largest or smallest member is not necessarily empty. Alternatively, being the smallest in the set of uninteresting numbers does not make a number interesting, at best it is unique like every other number and therefore uninteresting.
- TylerE 7y agoNot true of dates though, since you can set hard bounds at the big bang (or whatever) and the heat death of the universe.
- _morgs_ 7y agoSo the universe creates time?
- dkersten 7y agoYes. After the heat death, nothing will ever move or change and so it the universe will remain in a frozen state. Time will have stopped. Sine time is relative based on the observer and observed, if neither change in any way (no “work” happened and no information is exchanged, even on the level of fundamental particles (or fields/loops/strings or whatever happens to be at the bottom)), for every possible combination of observer and observer, then time has not advanced in any meaningful sense.
- deleted 7y ago[deleted]
- marcosdumay 7y agoYou don't need both bounds anyway. A countable set without a lower bound in any representation is already a paradox.
- fortran90 7y ago> There is no largest or smallest real number. The interesting number paradox applies to natural numbers, not real numbers. > Alternatively, being the smallest in the set of uninteresting numbers does not make a number interesting You are taking this too seriously. :-) Everyone would agree that "interesting" is subjective. This is supposed to be a humorous paradox, not a mathematical fact!
- ph2082 7y agoYes. I know. I Know. But you can't cut cake everyday. Need to make up some reason.
- lerie 7y ago2030-03-03
- skrebbel 7y agoI love the term "middle endianness" and how clearly it highlights how nuts the American date notation is. EDIT: fwiw, in my native language, most numbers from 100 and 1000 are pronounced middle endian, so I'm not complaining.
- thaumasiotes 7y agoThe American date notation is simply a direct reflection of the spoken language. We say "February second, 2020" and we write "2/2/2020". (Contra the root comment, I can't agree that 2/2/2020 is a palindrome.) I don't see this as being crazy or even unusual. When order matters, you invariably want big-endian notation, which is why that notation is acceptable everywhere regardless of the usual native notation. But by the same token, it means that all other styles are equally "nuts". European day-month-year notation doesn't have any technical advantages over month-day-year.
- justinclift 7y ago> We say "February second, 2020" ... Interesting. We (Australian) say "Second of Feb, 2020". ;)
- rags2riches 7y agoWhat about the Fourth of July then?
- thaumasiotes 7y agoWhat about it? It's the name of a holiday. 12/25 is often called Christmas, and yet 9/29 is never called Michaelmas. Same thing.
- mkagenius 7y agoAlso they contain only two distinct digits, rest mostly have 3 or more different digits.
- toyg 7y agoYeah, we won’t get a two-digit universal palindrome for a long time (12-12-2121).
- susam 7y agoYes, a YYYY-MM-DD format palindrome date that is also a palindrome in DD-MM-YYYY and MM-DD-YYYY formats cannot have more than two distinct digits. Let us represent the 8 digits of such a date (in YYYY-MM-DD format) as abcd-ef-gh. Since abcd-ef-gh (i.e., YYYY-MM-DD format) is a palindrome, we get a = h, b = g, c = f, and d = e. Since ef-gh-abcd (i.e., MM-DD-YYYY format) is a palindrome, we get a = h, b = g, c = f, and d = e. Since gh-ef-abcd (i.e., DD-MM-YYYY format) is a palindrome, we get a = f, b = e, c = h, and d = g. Combining all of these, we get a = c = f = h and b = d = e = g. Therefore, such a date must be of the format abab-ba-ba, i.e., it may have a maximum of only two distinct digits.
- philshem 7y agoFrom another comment on this thread > 20300302, 20400402, etc https://news.ycombinator.com/item?id=22214936 https://news.ycombinator.com/item?id=22214936
- susam 7y agoThey are not universally palindromic. For example, 2030-03-02 (yyyy-mm-dd) when written as 02-03-2030 (dd-mm-yyyy) is not a palindrome. All dates in the list I have provided are universally palandromic, i.e., they are palindromes irrespective of the endianness (big, little, or middle) used to represent them.
- Aperocky 7y agoWhere is 2021-12-02 though? edit: NVM. Should read better.
- susam 7y ago2021-12-02 (yyyy-mm-dd) is not a palindrome when written as 02-12-2021 (dd-mm-yyyy). The list in my comment contains only those dates that are palindromes in all of these three formats: yyyy-mm-dd, dd-mm-yyyy, and mm-dd-yyyy.