I guess 12 months grace period.

"If you didn't get Part B when you're first eligible, your monthly premium may go up 10% for each 12-month period you could've had Part B, but didn't sign up. In most cases, you'll have to pay this penalty each time you pay your premiums, for as long as you have Part B. And, the penalty increases the longer you go without Part B coverage.

Usually, you don't pay a late enrollment penalty if you meet certain conditions that allow you to sign up for Part B during a Special Enrollment Period. Read more about different situations that may affect when you decide to get Part B. "

https://www.medicare.gov/your-medicare-costs/part-b-costs/part-b-late-enrollment-penalty

 

So if the penalty is "monthly premium may go up 10% for each 12-month period", it would follow that if you enrolled on the 11th month, the premium won't go up?  

This is just pure speculation.