Age, Anniversaries & Life Dates

This category covers durations measured against a specific, personally meaningful date: how old someone is (today, or as of some other date), an exact breakdown into years, months, and days, how many days someone has been alive, and anniversaries that count forward from an event rather than a fixed calendar position.

A February 29 birthday needs a stated convention

Someone born on February 29 has an actual birthday only once every four years. In a non-leap year, treating that anniversary as February 28 or as March 1 are both used conventions, and they are not interchangeable. Any result touching this case states which convention it used rather than picking one silently, and none of these results are a legal determination of age for any purpose (a birthday calculation is not the same question as legal age, which depends on jurisdiction-specific rules this site does not attempt to resolve).

"Years old" hides a rounding decision

A person's age in whole years changes exactly once, on their birthday. Right up until that date each year, they are still their previous age, even if the current date is closer to the next birthday than the last one; age is a floor, not a round-to-nearest figure, and an exact breakdown (years, months, and days) makes that explicit instead of just stating one whole number.

Where to go next

For a plain date difference without the birthday-specific conventions above, see Date & Calendar Math. For a recurring anniversary reminder rather than a one-time calculation, see Recurrence & Events.