Every calculator here states the convention it used, because that is where calculators disagree. This page is the full list of those conventions in one place. If a number on this site surprises you, the explanation is almost certainly below.
Date arithmetic is calendar-local, never millisecond-based
Adding a day means incrementing the day component of a (year, month, day) triple. It does not mean adding 86,400,000 milliseconds. The two agree on 363 days of the year and disagree on the two days US clocks change: on the second Sunday in March a day is 23 hours long, and on the first Sunday in November it is 25. A calculator that adds milliseconds lands on the wrong calendar date across those boundaries, which is why some tools quietly disagree with each other twice a year. Ours cannot, because a clock change does not touch the day component.
The same rule runs through the whole site: the static answer baked into each page at build time and the live answer your browser recomputes both work on calendar components.
A date string is parsed as local, not UTC
new Date('2026-03-07') in JavaScript parses as midnight UTC, which is the
evening of 6 March everywhere in the Americas. A calculator that does that shows the wrong day to a
large slice of its visitors. We parse ISO dates into local (year, month, day) components explicitly,
and we reject impossible dates rather than rolling them over — 30 February is an error here, not
2 March.
The start date is not counted
Day 1 is the day after the start date. "30 days from today" means the date 30 days later, not the 30th day of a run that includes today. This is the usual convention for a deadline expressed in days, and it is the single most common source of an off-by-one between two calculators — so every generated page states it and shows the other number as well. Contracts, court rules and tax codes sometimes count the start day; check yours.
Counting between two dates follows the same rule: the end date is counted, the start date is not, so 1 January to 8 January is seven days. The days-between tool shows both figures.
Month and year arithmetic clamps at the month end
31 January plus one month is 28 February — 29 February in a leap year — not 3 March. We clamp to the last valid day of the target month rather than letting the overflow roll forward. This matches every legal and accounting definition of "one month" we are aware of, and it is what most people mean. The consequence worth knowing: the operation is not reversible. 31 January plus one month is 28 February, but 28 February minus one month is 28 January.
Years clamp the same way, which only shows up on one date: 29 February plus one year is 28 February.
The leap-year rule, in full
A year is a leap year if it divides by 4, except that a century year is not, unless it also divides by 400. So 2024 and 2028 are leap years, 1900 was not, and 2000 was. The 400 clause is why 2000 broke a generation of software that had only implemented the first two.
ISO 8601 weeks against the US convention
Two week-numbering systems are in common use and they disagree for part of most years.
- ISO 8601: weeks start on Monday, and week 1 is the week containing the first Thursday of January — equivalently, the week containing 4 January. A year has 53 ISO weeks when it starts on a Thursday, or when it is a leap year starting on a Wednesday.
- US convention: weeks start on Sunday, and week 1 is the week containing 1 January.
A worked disagreement: 1 January 2027 is a Friday. Under the US convention it is in week 1 of 2027. Under ISO 8601 it is in week 53 of 2026, because the week containing it has its Thursday in December. The week number calculator shows both, and every generated page states which one it is quoting — always ISO.
The business-day rule
A business day here is Monday to Friday, excluding the eleven US federal holidays on the dates they are observed. Three parts of that are choices, and each is one that competitors make differently:
- Which weekend. Saturday and Sunday. Not configurable, because the entire measured audience for these pages is US administrative and legal deadline counting.
- Which holiday set. Federal only. Not state holidays, not bank holidays, not court holidays, not company holidays — those vary by jurisdiction and by employer, and a calculator that silently picked one would be wrong for most visitors. See where our holiday dates come from.
- Observed dates. A federal holiday falling on a Saturday is observed on the preceding Friday; one falling on a Sunday is observed on the following Monday. It is the observed date that is skipped, because that is the day offices are shut. This means 1 January falling on a Saturday moves the closure into the previous December.
And, as everywhere else, the start date is not counted.
Holiday dates
Fixed and floating holidays are computed from their rules. Easter and the dates that hang off it come from the anniversary computus. Jewish observances come from the arithmetic Hebrew calendar and are exact. Islamic observances are computed from the tabular Islamic calendar and are estimates, because the real dates depend on a local moon sighting — those pages say so rather than presenting a computed date as settled. Equinoxes and solstices are computed astronomically and given in UTC, which matters: an equinox at 00:05 UTC is the previous evening in the Americas. Full detail, including the one date we carry with no algorithm behind it, is on the holiday sources page.
Time zone data
The static offsets on the zone pages are derived from the three daylight-saving rules the twelve zones we cover actually use — US, EU and southern-hemisphere Australian — checked against the IANA database at the version stated on each page. The live converter uses your own browser's copy of the IANA database, so if a government changes a rule the live answer follows it before we rebuild. Zone rules change by legislation, sometimes at short notice; the pages carry a review date for that reason.
The site rebuilds every morning
Several hundred pages here answer a question about "today", so they are regenerated daily and every one of them shows the date it was built. If you are reading a page whose rebuild date is not today, the number in the answer panel was correct on the date shown and your browser has already recomputed the live figure. A deploy that produces a wrong answer for a known offset fails the deploy rather than shipping.
Who runs this
See about this site for who is responsible for it and how to reach them. If a number here is wrong, that page is where to report it — we would rather fix it than argue about it.