Are you one of the people who find joy in discovering the small coincidences and symmetries, those mathematical Easter eggs the universe has laid out for us?
If so, make a big red X in your calendar, for we’ll experience a date that may give you goosebumps.
What Is It With September 27?
September 27 is many things—most of them not exactly worth writing home about. It’s the 270th day of the year. It’s a Saturday. And, not least, it also happens to be Crush a Can Day.
However, the date has an inherent beauty that reveals itself only if you take it apart mathematically: Not only is it a square date, it is a global square date.
2025 is also a square yearFinding September 27, 20252
If you write September 27, 2025, or 09/27/2025, as one number, you get 9,272,025. Turns out, that is a square number: 3045 x 3045 = 9,272,025
Ah, but what if you live in a country where the day comes before the month: 27 September 2025, or 27/09/2025? In that case, writing the date as one number gives you 27,092,025.
The fun fact is that this is also a square number: 5205 x 5205 = 27,092,025
Confused? Let math influencer Howie Hua explain it to you:
A Rare Occurrence
If this doesn’t quite get your heart racing, consider this: A date that translates into a square number like that in both date formats—i.e., a global square date—only happens eight times this century!
To compare, Blue Moons happen once every two or three years, while a solar eclipse happens somewhere at least once every six months.
The next global square date is on New Year’s Day 2036. However, in our humble opinion, that one is less impressive because both date formats result in the same number. It doesn’t matter if the day or the month comes first; the date is written 01/01/2036, and the resulting number is 1012036 in both cases. Compared to what awaits us on September 27, 2025, that is worth an honorable mention at best.
ISO 8601: The global date formatHow About September 1?
Aziz Inan, a professor of electrical engineering at the University of Portland, has calculated all occurrences of square dates in both date formats. Among the dates he flagged for both date formats are January 9 and September 1, 2025.
However, both of those dates require you to allow for leading zeros in one format while disregarding them in the other. For example, September 1, 2025, only results in square numbers if you write the date as 9/1/2025 in the month/day/year format and 1/09/2025 in the day/month/year format—notice the discrepancy between the 1 in the first notation and 09 in the second.
The last global square date that didn’t require that tweak was September 9, 2016, or 9/9/2016. That was also the first global square date this century. But again, as with January 1, 2036, the resulting number was the same in both date formats, making it slightly less noteworthy.
Calculate interesting numerical patterns in datesAll Global Square Dates This Century
After September 27, 2025, there will only be four more global square dates this century:
- September 9, 2016
- January 9, 2025
- September 1, 2025
- September 27, 2025
- January 1, 2036
- May 5, 2049
- February 2, 2084
- May 21, 2089
2025 Is Also a Square Year
The icing on the mathematical cake is that the year 2025 itself is also perfectly square.
Not only is it the square of the sum of all ten digits of the decimal numbering system, it is also the sum of the cubes of the ten digits. Don’t believe it? Let us show you!