The bug exists every time today's date is greater than the lowest total number of days in a month (28). So it's a bug for 3 days in January, (sometimes 1 day in February,) 3 days in March, 2 days in April, etc.
of those, however, there's more than one date in january which would error, so the total is more than 5. 6 or 7 depending on if it's a leap year
There is no limitation in setMonth() such that you can only set the month to "the month after."
I'm sorry, but I think you misunderstand this bug.
That is, the number of days of a given year where this bug applies are: 3 in January, 3 in March, 2 in April, 3 in May, 2 in June, 3 in July, 3 in August, 2 in September, 3 in October, 2 in November, and 3 in December.
3+3+2+3+2+3+3+2+3+2+3