A plain countdown: the target date is given directly
The simplest case — you type the target date yourself. Counting to 31 December 2026 from 14 September 2026 needs no extra logic beyond the subtraction itself: 108 days.
A birthday countdown: the target date recurs annually
Here the target date isn't fixed — it's "the next time this month and day occurs," which could be this year or next depending on whether it's already passed. For a 15 June birth date counted from 14 September 2026, that means the target is 15 June 2027, not 2026.
A retirement countdown: the target date is derived from a birth date plus an age
Here the target is a one-time future date, computed by adding a whole number of years to a birth date. For someone born 15 June 1990, retiring at 60: 1990 + 60 years = 2050, giving a retirement date of 15 June 2050 — 23.8 years away from 14 September 2026. For someone born 15 June 1960 retiring at 60, the same addition gives 15 June 2020 — a date already in the past relative to 14 September 2026, so the result correctly reports that retirement has already happened rather than showing a countdown.
Why this framing is useful
Recognizing that all three tools share one core operation makes it clear why they can be mixed and matched — a retirement countdown is really "find a date, then count to it," which is exactly what a birthday countdown and a plain countdown also do, just with different rules for finding that first date.