Your exact age — down to the day
Not just how many birthdays you have had. It works out the exact gap between your date of birth and today in years, months and days, then breaks the same span down into total weeks, days, hours and minutes. Those are two different questions and most people only ever get the answer to the first one.
Months are not a fixed length. Going from 31 January to 28 February is 28 days, but going from 30 April to 30 May is 30 days, and both are "one month". So calculating age in years and months means counting calendar boundaries, not dividing by an average. Anything that divides total days by 365 will drift by a day or more, and it gets worse the older you are.
A year is roughly 365.2422 days, which is why we add 29 February every four years. Then we skip it in century years unless they divide by 400 — so 1900 was not a leap year, 2000 was. If you were born on 29 February you technically get a real birthday once every four years, and most systems quietly roll you to 28 February or 1 March in between.
No account, no stored data. The date goes in, the number comes out, nothing is kept.