When can I sign up for Medicare and when does my coverage start?

Your first chance to sign up is the Initial Enrollment Period. It lasts 7 months. It starts 3 months before you turn 65. It ends 3 months after the month you turn 65 [1]. If you miss it, you may have to wait.

You may also pay a late penalty for as long as you have Part B [1]. The penalty grows the longer you wait [1]. After that, you can sign up from January 1 to March 31 each year. If you sign up in this General Enrollment Period, your coverage starts the month after you sign up [1]. If you sign up before the month you turn 65, Part B coverage starts the month you turn 65. If you sign up in the month you turn 65 or during the 3 months after, it starts the next month [1].

Open Medicare.gov, When does Medicare coverage start?, at https://www.medicare.gov/basics/get-started-with-medicare/sign-up/when-does-medicare-coverage-start and read the part quoted above. Your own plan or policy papers hold details these sources do not give: "Don't know yet" for your case.