Plain version: Part B is optional and carries a premium, so its rules are all about timing. Sign up in your window, or delay only for a reason Medicare accepts.

Initial Enrollment Period (IEP): seven months, made up of the three months before the month of first eligibility, that month, and the three months after. Example: turning 65 on August 15 gives an IEP of May 1 through November 30.

Automatic enrollment: a U.S. resident (other than in Puerto Rico) who becomes entitled to Part A is automatically enrolled in Part B unless he or she declines it. Someone already receiving Social Security benefits becomes entitled to Part A at 65 without doing anything, and gets notice and time to decline Part B.

Delaying while covered by an employer plan: the CDI outline says Part B enrollment may be rejected or delayed without penalty while the person is covered by any employer-sponsored health plan. Under current rules (42 CFR §407.20(c)), for a person 65 or older the group coverage must be based on the current employment of the person or a spouse, so a retiree plan does not qualify. Months of qualifying coverage are excluded when the penalty is figured.

Special Enrollment Period (SEP): an eight-month window. It ends on the last day of the eighth consecutive month in which the person is at no time covered by a group plan based on current employment. Example: job and coverage end March 31, so April is month 1 and the SEP ends November 30.

General Enrollment Period (GEP): January 1 to March 31 each year, for people who missed their IEP and have no SEP. For enrollment on or after January 1, 2023, coverage begins the first day of the month after the person enrolls. (Under the old rule coverage waited until July 1, so older study materials may be out of date.) A late enrollment penalty may apply.