To the day — plus what weekday you were born and how far away the next birthday is.
Calendar-style: whole years first, then whole months, then leftover days — the same way official forms count age. Someone born March 31 turns “1 month old” on April 30, since April has no 31st.
Your birthday is counted as February 28 in non-leap years — the convention most jurisdictions use. Every fourth year you get the real one back.
Entirely in your browser. Dates you type are never sent anywhere — there is no server doing the math, no analytics watching you do it.