Those divisions are already skewed in a 12 month calendar though because the number of days is not similarly divisible:
Half year (6 months)
1st half: 181 days (182 for leap years)
2nd half: 184 days
Quarter year (3 months)
1st quarter: 90 days (91 for leap years)
2nd quarter: 91 days
3rd quarter: 92 days
4th quarter: 92 days
A leap year has 366 days which is evenly divisible by 2 and yet even then a “half year” at the monthly level doesn’t contain half the days of the year. Having uneven yearly divisions in a 13 month calendar would not be a new problem.
Those divisions are already skewed in a 12 month calendar though because the number of days is not similarly divisible:
Half year (6 months) 1st half: 181 days (182 for leap years) 2nd half: 184 days
Quarter year (3 months) 1st quarter: 90 days (91 for leap years) 2nd quarter: 91 days 3rd quarter: 92 days 4th quarter: 92 days
A leap year has 366 days which is evenly divisible by 2 and yet even then a “half year” at the monthly level doesn’t contain half the days of the year. Having uneven yearly divisions in a 13 month calendar would not be a new problem.