If you sign up for Medicare Part A (Hospital Insurance) and/or Medicare Part B (Medical Insurance) during the first 3 months of your Initial Enrollment Period, your coverage starts the first day of the month you turn 65. If your birthday is on the first day of the month, your coverage starts the first day of the prior month.
You can sign up for free Part A (if you’re eligible) any time during or after your Initial Enrollment Period starts. If you sign up within 6 months of your 65th birthday, your coverage will start at one of these times:
- The first day of the month you turn 65
- The month before you turn 65 (if your birthday is the first of the month)
After that, your coverage start date will go back (retroactively) 6 months from when you sign up.
For Medicare Part B (and for those who must pay a premium for Medicare Part A), the effective date of coverage depends on when you sign up.
Source: Medicare.gov