September 10 this year marks start of 10 back-to-back palindrome days

MANILA, Philippines (Update 2: June 29, 2020, 10:08 a.m.) —  September 10 this year marks the beginning of 10 consecutive palindrome days, the last one in a nine-year series that happens every century.

A palindrome day happens when the date can be read the same way forward and backward, calendar and time conversion site Time and Date said.

But this phenomenon of having successive palindrome days can only happen if you use a month-day-year format where you don't add a zero before single-digit months and only the last two digits of the year are written out. Such a format, commonly used in the United States, will result in the following palindrome days:

• 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

The next time we'll have 10 back-to-back palindrome days using the same month-day-year format will be from Jan. 20-29, 2021.

But using a date format where you add a zero before a single-digit month and include only the last two digits of the year will result in a palindrome week as soon as Feb. 1, 2020:

• 02/1/20
• 02/2/20
• 02/3/20
• 02/4/20
• 02/5/20
• 02/6/20
• 02/7/20
• 02/8/20
• 02/9/20

You can check this cheat sheet for the full list of palindrome dates that use a two-digit year format from 2001 to 2045.

Some palindrome weeks are common

Although encountering back-to-back palindrome days may be intriguing, it is actually not that rare in the month-day-year format used above.

And there's a pattern to it, according to Time and Date.

"As long as you write your date in the m-dd-yy format, every century has 9 years with 10 Palindrome Days in a row. These years are always in the second decade of the century. For example, every year between 2011-2019, 2111-2119, and 2211-2219 will have 10 consecutive Palindrome Days. This is true for previous centuries as well," it explained.

This sequence of palindrome days begins on the 10th and ends on the 19th in months whose number matches the last digit of the year. 

Simply put, we have had 10 consecutive palindrome days for each year since 2011. These occurred from the 10th to 19th of January for 2011 (1-10-11 to 1-19-11), February for 2012 (2-10-12 to 2-19-12), March for 2013 (3-10-13 to 3-19-13) and so on. It will happen in September for 2019. 

But some are unique

The website said that depending on date formats certain palindrome days could be rare.

It cited University of Portland electrical engineering professor Aziz Inan's calculations which found that in the month-day-year format where zero is added before single-digit months and all the digits of the year are written out, palindrome days occur only in the first few centuries for every 1,000 years. 

"... in the mm-dd-yyyy format, the first Palindrome Day in the current millennium (January 1, 2001 to December 31, 3000) was October 2, 2001 (10-02-2001) and the last such day will be September 22, 2290 (09-22-2290). There are 12 Palindrome Days in the 21st century in the mm-dd-yyyy format."

Meanwhile, adopting a day-month-year format where all digits of the year are written down, used mostly in the European languages, will produce 29 palindrome days in the current century.

"The first was 10 February 2001 (10-02-2001). The last is a special one – it's a leap day! 29 February 2092 (29-02-2092) will be the last Palindrome Day of the 21st century."

Editor's note: The story initially stated that it will be the last palindrome week of the century for the date format above instead of this decade.