Put the same two dates into three different calculators and you can get three different answers. That is not usually a bug in any of them. It is that several of the questions people ask about dates do not have one correct answer, and each tool has quietly picked one without telling you which.
The clearest case is the one that sounds least controversial: adding a month.
A month is not a length
Seconds, minutes, hours and days are durations: they have a size that does not depend on when you start. Months are not. February is 28 days, or 29; the rest are 30 or 31. So "one month later" is not arithmetic on a number line — it is a move on a grid, and the grid has ragged edges.
That is why the three answers above all exist. Clamping keeps you in February, which feels right if you think of it as "the same sort of day next month". Overflowing gives 3 March, which feels right if you think of dates as numbers that carry. Adding thirty days is a genuine duration, and lands somewhere else again.
The practical consequence is worth carrying around: if a deadline is expressed in months and it matters, find out which rule applies. Contracts and statutes usually say. Software usually does not.
Why your age is not days divided by 365
Ages are counted in anniversaries, not in elapsed time, and the two do not line up.
Somebody born on 20 May 1990 has been alive 13,260 days as of today. Divide by 365 and you get 36.33; divide by 365.25 and you get 36.30; the answer everybody actually uses is 36, because that is how many times the date has come round. The fraction is not "how far into your thirty-seventh year you are" in any of those calculations, and the three disagree with each other about it.
The Gregorian year averages 365.2425 days — 146,097 days in every 400 years, exactly — but no individual year is that long, so any division is an approximation of something the calendar was not doing in the first place. An age calculator that reports years, months and days is counting the grid, which is what a birth certificate does too.
The 29 February problem
Somebody born on 29 February has a birthday in 2024 and 2028, and none in 2025, 2026 or 2027. So on 28 February 2027, are they a year older or not?
There is no universal answer. Different jurisdictions land differently for legal purposes — some treat 1 March as the anniversary, some 28 February — and software mostly just picks one. If a leap-day date is doing legal work, it is worth checking rather than assuming, and if it is doing social work, the person concerned has almost certainly already decided.
"Between" is not one operation either
From Monday the 2nd to Friday the 6th is five days if you count both ends and four if you count only the gap. Both are right; they answer different questions. "How long am I away" counts both, "how many days until" counts neither.
Add weekends and public holidays and the same range yields several more legitimate numbers. That is why the working days calculator here asks which convention you want and prints the one it used underneath the answer, instead of picking silently and being the fourth tool that disagrees with the other three.
A date is not an instant
"8 September" is not a moment in time. It is a moment plus a place — and while it is the 8th in Buenos Aires it may already be the 9th in Tokyo. Anything with a deadline of "the 15th" has an unstated timezone in it, and the unstated one is usually the sender's.
Worse, the gap between two places is not constant. Clocks move twice a year and not on the same weekend, so between the two changes the usual difference is an hour out. Measured across 2025, 2026 and 2027, London and New York spend 21, 21 and 14 days each spring at four hours apart instead of five. The time zone converter warns when a clock change falls near the date you are asking about, which is the window where meetings go wrong.
The 23-hour day
One more consequence of the same thing, and the one that produces silent bugs rather than arguments. On a clock-change weekend a local day is 23 or 25 hours long. Anything that counts a range by adding 24 hours at a time to a local date will therefore skip or repeat a day, twice a year — in March and October, which are exactly the months people count their holiday allowance in.
It is invisible when it happens, because the total is only wrong by one and nobody recounts. Tools that work in universal time do not have the problem at all, which is why the counting here does.
What to do about all this
Three habits cover most of it. When a deadline is in months, ask what happens at the end of the month. When a range is quoted in days, ask whether the last day counts. And when a time crosses a border, say the zone out loud — "3pm London" costs four characters and removes the whole class of problem.