You've likely heard that palindrome sentence that is spelled the same way forwards and backwards. But palindrome dates seem to spark a lot of curiosity and discussion.

Palindrome dates, by their very nature, only occur in the early centuries of a millennium. There will be 36 of them during this millennium — the last one will be on Sept. 22, 2290. The next one after that won't be until Oct. 3, 3001.

In 2019, for example, every day for 10 consecutive days starting Sept. 10 is a palindrome in the m-dd-yy format, which is common in the United States:

• 9-10-19
• 9-11-19
• 9-12-19
• 9-13-19
• 9-14-19
• 9-15-19
• 9-16-19
• 9-17-19
• 9-18-19
• 9-19-19

Palindromic dates can be rare, depending on how a date is formatted.

Aziz S. Inan, an electrical engineering professor at the University of Portland, has calculated that when dates are written in the mm-dd-yyyy format, palindrome days typically occur only in the first few centuries of each millennium. The last palindromic date in the second millennium (1001 to 2000) in this format was Aug. 31, 1380 or 08-31-1380, according to timeanddate.com. The first one in the current millennium (Jan. 1, 2001 to Dec. 31, 3000) was Oct. 2, 2001 (10-02-2001) and the last one will be Sept. 22, 2290 (09-22-2290).

For countries that use the dd-mm-yyyy format, there are 29 palindrome days in the current century. The first was 10 February 2001 (10-02-2001). The last will be a leap day: 29 February 2092 (29-02-2092), which will also be the last palindrome day of the 21st century.

## Palindrome weeks

Although it might seem like strings of palindrome dates — or palindrome weeks — might be rare, Inan says that's hardly the case.

Since 2011, every year has had 10 consecutive palindrome days. In 2011, they started on Jan. 10 (1-10-11 through 1-19-11), for example, and in 2012, another string started on Feb. 10 (2-10-12 through 2-19-12). In 2017, it happened in July, while in 2018, it happened in August.

In the m-dd-yy format, every century has nine years with 10 consecutive palindrome days. Timeanddate.com points out that they're always in the second decade of the century. Every year between 2011-2019, 2111-2119, and 2211-2219 will have 10 palindrome days in a row.

But palindrome dates aren't the only way that calendar number geeks — I mean enthusiasts — get their thrills.

Among other patterns, there are repeating dates (1/11/11 = 11111), repeating sequences (10/31/03 = 103 103), and sequential dates (8/9/10 = 8,9,10; and if you start with the time of 12:34:56.7, you get 1, 2, 3, 4, 5, 6, 7, 8, 9, 10).

Editor's note: This story has been updated with new information since it was published in October 2012.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

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What are palindrome dates?
Numerical palindromes can be read the same forwards and backwards.