Date & Age Calculator
Days between two dates, counted both ways — because the one-day argument is almost always about which convention you mean.
Why two calculators give two answers
Almost every "off by one" argument about dates is really a disagreement about whether the endpoints count. Both conventions are standard; they are just used in different places.
"A month" is not a length
The calendar-difference row above is expressed in years, months and days because that is how people actually think about spans — but it hides an ambiguity worth knowing. Months run from 28 to 31 days, so "one month after 31 January" has no single correct answer: clamp to the end of the month and you get 28 February, add 31 days and you get 3 March, and both conventions are in use. Anything with a deadline attached — a notice period, a return window, a warranty — is worth reading carefully rather than trusting to a calendar app's default.
The rule that catches software out
A leap year is any year divisible by four — except centuries, unless those are divisible by four hundred. The third clause is the one people drop, and dropping it is why a spreadsheet full of 1900 dates still runs a day out.
Questions
Why do date calculators disagree by one day?
Because they answer different questions. Subtracting one date from another counts the gaps between them; counting both the start and end day counts the days themselves. Monday to Friday is four by subtraction, five inclusive. Neither is wrong — hotels, sentences and leave entitlements usually count inclusively, while age and interest normally do not.
How are leap years handled?
By the full Gregorian rule: a year is a leap year if divisible by 4, except centuries, unless divisible by 400. So 1900 was not a leap year and 2000 was — a distinction that broke a lot of software. This calculator applies the rule rather than approximating a year as 365.25 days.
How is age calculated?
By calendar, not by dividing days by 365.25. You are 30 on the day your thirtieth birthday arrives, regardless of how many leap days fell in between — which is how ages are treated legally and medically in most countries.
What does 'one month later' actually mean?
There is no single answer, because months are not a fixed length. One month after 31 January can reasonably be 28 February, 2 March or 3 March depending on the convention, and different software picks differently. When a date matters — a notice period, a warranty, a payment — read how the contract defines it rather than assuming your calendar app agrees.
References
Reviewed September 2026.