The start of Medicare coverage depends on whether you choose the Initial Enrollment Period or the General Enrollment Period, explains Medicare.gov. If you enroll during the IEP, the date your coverage starts depends on your birthday and enrollment date; if you enroll during the GEP, coverage starts on July 1.
If you enroll for Medicare Part A and/or Part B during the IEP, the main determinant of the start of your coverage is your birthday date, Medicare.gov states. If you enroll before your 65th birthday, coverage will start on the first day of the month of your birthday; the only exception is if your birthday is on the first of the month. In that case, coverage starts on the first day of the preceding month; for example, if your birthday is on May 1, your coverage will start on April 1 if you enroll in January, February or March. If you enroll after your 65th birthday, your coverage date depends on how much time has passed since your birthday. Coverage will start a month after enrollment if you have enrolled in your birthday month, two months after enrollment if you have enrolled a month after your birthday, and three months after enrollment if you have enrolled two to three months after your birthday.