There is a list of "Sub-year groupings" which assign two-digit numbers to seasons (hemisphere independent), seasons (Northern Hemisphere), seasons (Southern Hemisphere), quarters, quadrimesters (four-month periods), and semestrals (six-month periods). The numbering starts at 21.
The numbers 1 to 12 are already used to represent months in formats such as YYYY-MM, so starting this list higher than 12 allows these (certainly rare) representations to coexist. e.g. "2021-12" represents "October, 2021" and "2021-29" represents "Spring, Southern Hemisphere, 2021".
The numbers 1 to 12 are already used to represent months in formats such as YYYY-MM, so starting this list higher than 12 allows these (certainly rare) representations to coexist. e.g. "2021-12" represents "October, 2021" and "2021-29" represents "Spring, Southern Hemisphere, 2021".