If I earn my 40th Social Security credit mid-year, is my Medicare Initial Enrolment Period when I earn the credit or when the credit is filed for?

I’m a doozy of an edge case. I’m above 65 and will become eligible for Medicare mid-year 2024, which is when I’ll have earned my fortieth Social Security credit on Form 1040 Schedule C. (I won’t have qualified before then because I won’t yet have resided in the USA for five consecutive years.)

Is my seven-month Medicare Initial Enrolment Period determined by the date in 2024 by which I’ve earned enough to get my fortieth Social Security credit? Or is it determined by when I file my Form 1040 Schedule C in April 2025? Or something else?