Most retirement timelines give you ages. Ages are the easy part — what you actually need is the month. One date of birth below, and every deadline comes back as a real date, with the rule that produced it.
Social Security treats you as reaching an age the day before your birthday, which is why someone born on 1 June reaches 62 on 31 May. The IRS does not use that rule for the 59½ date — that one runs six calendar months from your actual birthday. Medicare has a third rule of its own, which moves a first-of-the-month birthday back a month. Three agencies, three ways of counting, and each date above says which one it used.
Here is what that costs in practice. Someone born on 1 June 1963 and someone born on 2 June 1963 can start Social Security in the same month — both are 62 for the whole of June 2025. But their Medicare windows are a month apart, February to August 2028 against March to September 2028, because Medicare moves the 1st back and Social Security does not. One day of difference, and the two agencies disagree about which month you turned 65.
None of this is rounding. Before full retirement age you have to be that age for a whole month before a benefit can be paid for it, so a single day either side of a month boundary is a month of payments.
Estimates and general information only — not tax, investment or financial advice, and not affiliated with the Social Security Administration, the IRS, Medicare or any government agency. Dates follow the rules as published today and change when the law changes.