Two correct answers
"How old is this person" has a calendar answer and a duration answer, and both are shown because different things need each:
- Calendar — 36 years, 3 months, 13 days. What a person says.
- Total days — 13,254. What an interest calculation, an insurance table or a medical dosage uses.
Neither converts cleanly into the other, because months are not a fixed length.
The borrowing rule
When the day of the reference date is earlier in the month than the day of birth, a month is borrowed and its real length is added. From 31 January to 1 March borrows February's 28 or 29 days. This is what makes the answer match what a person would say out loud, rather than what a naive division produces.
Why the timezone handling matters
Subtracting two local timestamps to get a day count breaks whenever daylight saving falls between them — one of those days is 23 or 25 hours long and the division rounds the wrong way. Counting at UTC midnight avoids the problem entirely, which is why an autumn span here does not come back a day short.
Questions
How are months counted?+
The way a person counts: whole calendar months, borrowing days from the previous month when needed. From 31 January to 1 March is one month and one day, not 29 days divided by 30.
Does it handle leap years?+
Yes. Days are counted at UTC midnight so a leap day is an ordinary day and a daylight saving change cannot shift the total. Someone born on 29 February has their birthday counted on 1 March in common years.
Can I calculate age on a past or future date?+
Yes — set the reference date to anything. That is what makes this useful for eligibility checks, insurance calculations and working out how old someone was when something happened.
Why does the total days figure differ from years × 365?+
Because of leap years. Over 36 years there are nine extra days. Multiplying by 365 undercounts, and by 365.25 overcounts for most spans — counting real calendar days is the only accurate method.