Subtracting two dates sounds like arithmetic. It is arithmetic on a calendar built from compromises between astronomy and politics, which is why "how old is this person" has more than one defensible answer and why date libraries are so much bigger than you would expect.
Why years, months and days is not one number
This tool reports an age two ways, and they are answering different questions. Years-months-days walks the calendar: how many whole years fit, then whole months, then the remainder in days. Total days counts the actual elapsed days. Neither converts cleanly to the other, because months are not a fixed length.
The calculation borrows the way subtraction does on paper. If the day-of-month has not come round yet, it borrows the length of the previous month — which is 28, 29, 30 or 31 depending on where you are standing. That is why "1 month and 3 days" means a slightly different span in February than in July, and why totalDays divided by 30 will never quite agree with the months figure.
The 29th of February has no universally correct answer
Someone born on 29 February has a birthday that exists once every four years. Every calculator must decide what to do in the other three, and there is no answer that is simply right.
This one inherits JavaScript's behaviour: asking for 29 February 2026 produces 1 March 2026, because the date rolls forward when the day does not exist. So a leap-day birthday shows its next occurrence as 1 March in non-leap years. That is a defensible choice and it matches how a lot of people actually celebrate — but it is a choice, not a fact.
The law does not agree with itself either. In the United Kingdom, a person born on 29 February legally attains their age on 1 March in common years. In much of continental Europe the convention lands on 28 February. Taiwan says 28 February; Hong Kong says 1 March. Same birth date, same calendar, different legal answer depending on where you are standing.
The age in whole years is unaffected — someone born on 29 February 2000 is 25 in July 2025 under every convention. It is only the exact day of the transition that is disputed, and only for one person in about 1,461.
Month arithmetic is ambiguous, not just fiddly
Ask what 31 January plus one month is and there is no correct answer. 28 February preserves "end of month" but loses three days. 3 March preserves the interval but breaks the intuition that adding a month should land in the next month. Different libraries make different choices, and both are wrong for somebody.
This is worth knowing because it shows up in things that matter — subscription renewals, contract terms, notice periods. If you are computing a date that has money attached to it, find out what convention the contract intends rather than what your date library happens to do, because they are not obliged to agree.
The calendar itself has a hole in it
For dates far enough back, "how many days ago" stops being a well-formed question. The Gregorian calendar replaced the Julian one in 1582, and ten days were simply deleted — in Catholic countries, 4 October was followed by 15 October. Those days do not exist.
Different countries switched at wildly different times. Britain and its colonies held out until 1752, by which point the gap was eleven days; Russia switched in 1918, Greece in 1923. So the elapsed days between two historical dates depends on where the dates were recorded, which is why genealogists write things like "O.S." and "N.S." beside dates. Every ordinary date calculator, this one included, projects today's Gregorian calendar backwards forever and quietly ignores all of it. For dates within a lifetime it does not matter. For anything before the eighteenth century, it does.
Common questions
Is my date of birth sent to a server?
No. This tool is marked "client": the arithmetic happens in your browser tab and the date is not transmitted or stored. Worth stating for this tool in particular — a date of birth is one of the standard identifiers used to verify who you are, which makes it worth more than it looks.
Why does the total days not equal years times 365?
Leap years. Over 25 years there are six or seven of them depending on where the dates fall, so the total drifts by about a week from the naive multiplication. A year is 365.2425 days on average, which is itself an approximation chosen to keep the calendar aligned with the seasons — the reason for the awkward rule that century years are not leap years unless divisible by 400.
Does it handle timezones?
It works in your local calendar, deliberately. A date of birth is a date, not an instant — you were born on a day, in a place, and that day does not shift because someone in Tokyo is reading the page. Treating a birthday as a timestamp is what produces the classic off-by-one where someone's age changes at 1am, and this avoids it by never converting to an instant at all.
Which day is a leap-day birthday, legally?
It depends on the jurisdiction, and there is no international answer. The UK says 1 March, much of continental Europe says 28 February, and it varies elsewhere. For anything with legal weight — coming of age, a contract term, an eligibility cutoff — check the rule that applies where it matters. This tool shows 1 March, which is one of the two common answers, not the universal one.
Can I calculate an age at a past or future date?
Yes — the reference date defaults to today but you can set it to anything. That is the useful case for eligibility questions: how old will this person be on the first day of term, or were they old enough on the date of the contract. Both are questions about a date that is not today, and both are the reason the field exists.