The 2021 calendar of the 21st century is rich with mathematical curiosities. Some of the interesting ones discovered are listed below:

• 2021 is the 21st year of the 21st century.

• The left and right halves of 2021, namely 20 and 21, are consecutive numbers, a property which repeats in four-digit years once every 101 years.

• Additionally, 2021 equals the product of two consecutive prime numbers, 43 and 47. The last year this happened was 1763 (1763 = 41 x 43) and the next one after 2021 will be 2491 (2491 = 47 x 53).

• 2021 contains two full palindrome dates: Jan. 20, 2021, expressed with seven digits as 1/20/2021 and Dec. 2, 2021, expressed with eight digits as 12/02/2021. 2011 is the only other year in this century that has two full palindrome dates: 1/10/2011 and 11/02/2011.

• Moreover, 1/20/2021 and 12/02/2021 are the 14th and 15th of the total 38 full palindrome dates in the 21st century. Coincidentally, the 14th and 15th prime numbers are 43 and 47, and 43 x 47 = 2021.

• If the year part of a date number is expressed in terms of the rightmost two digits of the year, 2021 will offer a total of 22 palindrome dates. Interestingly, eleven of these 22 palindrome dates will occur in January: 1/2/21, 1/20/21, 1/21/21, 1/22/21, 1/23/21, 1/24/21, 1/25/21, 1/26/21, 1/27/21, 1/28/21, and 1/29/21. The other 11 will occur in December: 12/1/21, 12/2/21, 12/3/21, 12/4/21, 12/5/21, 12/6/21, 12/7/21, 12/8/21, 12/9/21, and 12/11/21. Note that the only other year in the 21st century that also contains 22 palindrome dates is 2011.

• Palindrome date 1/20/21 is also special since 1 + 20 = 21.

• Jan. 9, 2021, expressed as 1/9/2021 consists of three consecutive numbers: 19, 20, and 21.

• April 1, 2021, written as 4/1/2021 is special too since 41 = 20 + 21.

• April 3, 2021, expressed as 4/3/21 consists of sequential numbers 4, 3, 2, and 1, in decreasing order.

• April 20, 2021, expressed as 4/20/2021 is also special since 420 = 20 x 21.

• Aug. 13, 2021, written as 8/13/21 consists of three consecutive Fibonacci numbers: 8, 13, and 21.

• Aug. 19, 2021, expressed as 8/19/2021 is special since 2021 minus 819 equals 1202, the reverse of 2021. The last time this happened was on 9/09/2011 since 2011 - 909 = 1102 and it will happen again on 7/29/2031.

I hope you enjoy the special numerical characteristics of 2021 throughout the year. Happy New Year!

*Aziz Inan is a professor teaching in the electrical engineering program of the Donald P. Shiley School of Engineering at University of Portland and he can be reached at **This email address is being protected from spambots. You need JavaScript enabled to view it.**.** *

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