Dear Savvy Senior,

What can you tell me about the different enrollment periods for Medicare? I’m planning to work past age 65 and understand Medicare offers initial, special and general periods in which I can enroll. How does this work? — Medicare Illiterate

Dear Medicare,

The rules for signing up for Medicare can be quite confusing, especially if you plan to work past age 65. But it’s critical to understand the ins and outs of enrolling because the consequences of missing a deadline can be costly and last a lifetime.

Here’s what you should know about Medicare’s three different enrollment periods.

Initial enrollment period

At age 65, the initial enrollment period is the first opportunity for most people who are eligible to enroll in Medicare.

If you’re already claiming Social Security benefits at least four months before age 65, you are automatically enrolled in Medicare, with coverage starting the first day of the month you turn 65. If you’re not receiving Social Security benefits, it’s up to you to enroll in Medicare either online at SSA.gov/Medicare, over the phone at 800-772-1213 or through your local Social Security office.

You can enroll any time during the initial enrollment period, which is a seven-month period that includes the three months before, the month of and the three months after your 65th birthday.

It’s best to enroll three months before your birth month to ensure your coverage starts when you turn 65.

If, however, you plan to keep working and have health coverage from your employer, or from a spouse’s employer, you may want to delay Medicare Part B, which covers outpatient services, and Part D, which covers prescription drugs.

But first check with the human resources department to see how your employer insurance works with Medicare.

Typically, if your employer has fewer than 20 employees, Medicare will be your primary insurer and you should enroll. But if you work for a company that has 20 or more employees, your employer’s group health plan will be your primary insurer as long as you remain an active employee.

If this is the case, you don’t need to enroll in Part B or Part D when you turn 65 if you’re satisfied with the coverage you are getting through your job.

But in most cases, unless you’re contributing to a health savings account, you should at least sign up for Medicare Part A, which is free and covers hospital services.

Special enrollment period

If you delay Part B and Part D past age 65, you can sign up for Medicare during the special enrollment period. Once you (or your spouse) stop working and you no longer have group health coverage, you have eight months to enroll in Part B. But if you miss that deadline, you’ll pay a late-enrollment penalty for the rest of your life. The penalty increases your premiums by 10% for each 12-month period that you don’t have coverage.

The window for Part D is shorter. You must sign up for Part D within two months of losing drug coverage. If you go 63 days or more without drug coverage, you’ll pay a lifetime late-enrollment penalty that equals 1% of the monthly base premium (about $33 in 2019) times the number of months you don’t have Part D of other creditable coverage.

General enrollment period

If you miss either of these first two enrollment periods, you’ll have to wait until the general enrollment period, which is Jan. 1 through March 31 of each year, but your Part B and Part D coverage will not begin until July 1. And you’ll be subject to late-enrollment penalties.

There is, however, no penalty for late enrollment for Part A. You can sign up anytime with coverage beginning the first day of the following month.

Jim Miller is a contributor to NBC-TV’s “Today” program and author of “The Savvy Senior.” Send your questions to Savvy Senior, P.O. Box 5443, Norman, OK 73070; or visit savvysenior.org.