And then we have a prime number of months. Checkmate OP

Congrats you invested invented International Fixed calendar - https://en.wikipedia.org/wiki/International_Fixed_Calendar

Edit: typo

FYI, there is kind of a push toward this, usually by landlords and other bill collectors, who would just love to be able to charge an extra month of rent or bills each year.

There's no way in Hell under the current leadership in many parts of the world that bills would be multiplied by 12, then divided by 13, and then capped at that. The cap wouldn't work anyway. The only responsible way to do it would be to charge bills on an annual basis, and divide it by the "number of months" for the "convenience" of the consumer. Then it would just be set up to where all the monthly payments would go down. But yeah, no way they enforce that.

That's metric months.