> Having a 13 month calendar simplifies so much from financial calculations to having consistent day of week/month boundaries that I can't see any real down sides.
It seems like that would complicate the business practice of dividing your fiscal year into quarters. I guess each quarter would be 3 months + 1 week but then you don't have quarters starting/ending evenly with months.
Not really. You have 13 months, but you have 4 weeks each month. The first quarter is 3 months and 1 week. Then 6 and 2, and 9 and 3. The final quarter ends at the end of the year.
Today if you pay a monthly fee for something, the daily cost varies month-to-month. Why should things cost the most in February? While it may be true that fiscal quarters won't be aligned with month boundaries, they are more evenly spaced.
It seems like that would complicate the business practice of dividing your fiscal year into quarters. I guess each quarter would be 3 months + 1 week but then you don't have quarters starting/ending evenly with months.