A calendar quarter is a three-month period of time ending with March 31, June 30, September 30, or December 31. Social Security counts each calendar quarter that you work and pay into Social Security and Medicare taxes toward your eligibility for premium-free Part A. Calendar quarters can also refer to the time periods when people with Extra Help have a Special Enrollment Period (SEP) to change their Part D or Medicare Advantage drug coverage, which is once per quarter in the first three calendar quarters of the year.