Ask two date calculators how many days lie between the first of March and the first of April and you will get 31 from both. Ask how many days there are from 31 January to 1 March and one of them may say 29 and another 30. Neither is broken. They are answering slightly different questions, and understanding which question is which saves you from a surprising number of paperwork and planning errors.
If a project starts on Monday and ends on Friday, is it four days long or five? Subtraction says four (Friday minus Monday). Counting the days you actually work says five. A date-difference calculation is normally exclusive of the start date: it measures the number of midnights crossed. A hotel stay works the same way. Check in on the 10th, check out on the 13th, and you pay for three nights even though you touched four calendar dates.
Contracts that say "within 30 days" often mean 30 days after, not including, the trigger date. Contracts that say "for a period of 30 days commencing on" may include it. When it matters, read the wording, and if you build the number in a tool, note which convention the tool uses. The day counter and the date calculator both measure the span between two dates, so you can test both readings.
The full rule has three parts: a year is a leap year if it is divisible by 4, except if it is divisible by 100, unless it is also divisible by 400. So 1996 and 2024 were leap years. 1900 was not. 2000 was. 2100 will not be.
The reason is that the true length of the solar year is about 365.2422 days. Adding a day every four years assumes 365.25, which overshoots. Dropping three leap days every 400 years brings the calendar average to 365.2425 days, which is close enough to drift by only about a day in several thousand years. In a 400-year cycle there are 97 leap years, and 97 / 400 more days per year gives you that 0.2425.
This calendar, the Gregorian one, was introduced in October 1582, and ten days were skipped to correct accumulated drift. Many countries adopted it much later. Britain and its colonies switched in 1752, dropping eleven days. If you are working on historical dates, a calculator that applies the modern calendar backward gives answers that differ from how people at the time dated their documents.
Take someone born on 1 January 2000. On 6 October 2026, they have been alive for 9,775 days. That span contains seven leap days: 29 February in 2000, 2004, 2008, 2012, 2016, 2020 and 2024. Their 10,000th day will land on 19 May 2027, a bit under eight months later.
Now take someone born on 15 March 1990. On 6 October 2026 their age is 36 years, 6 months and 21 days. The logic: from 15 March 1990 to 15 March 2026 is 36 years. From 15 March to 15 September is 6 months. From 15 September to 6 October is 21 days. The age calculator produces this breakdown, and it is worth testing a known case like this yourself before trusting any tool on a legal date.
Months have different lengths, so "one month later" is ambiguous at the end of a month. What is one month after 31 January? There is no 31 February. Some systems return 28 February (or 29 in a leap year), others roll forward to 3 March, and some report an error. Note that 31 January to 28 February is 28 days, while 31 January to 1 March is 29 days in a common year. Subscription renewals, rent due dates and loan schedules all depend on which convention the software uses, so if a billing date seems to wander from the 31st to the 28th and never comes back, this is the reason.
For anything that matters, count in days or weeks, which are unambiguous, and convert to months only for display.
A person born on 29 February turns one on a day that does not exist for three years out of four. Counted in days, someone born on 29 February 2000 had lived 9,716 days by 6 October 2026. Legal systems differ on which day counts as the birthday in common years: some treat 28 February as the legal date, some 1 March. This can matter for things like the day someone becomes eligible to drive or vote, so check the rule for the jurisdiction in question instead of assuming.
Most of the world counts age as completed years since birth. Traditional Korean age counting is different: a baby is one at birth and everyone gains a year on 1 January. A child born on 31 December would be two years old the next day by that count, while the international count would call them a day old. In June 2023 South Korea moved most legal and administrative purposes to the international system, though some rules still key off the year of birth alone. Other East Asian traditions had similar approaches, which is why forms for international students and travelers sometimes ask for age "in years, international count".
A birth at 11:30 pm on 31 December in one time zone is already 1 January in another. For day counting, the local date at the birth location is normally what counts, and most calculators assume you give it as written on the certificate. For software that stores timestamps, converting to UTC and then back to a local date can shift a date by a day, a classic bug in age checks near midnight.
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