How the calculation works
Pick a start date, choose a unit and enter a number. The calculator moves forward for positive numbers and backwards for negative ones, then reports the resulting date along with its day of the week, its ISO week number and its day-of-year number.
All arithmetic is calendar-local. That matters more than it sounds: adding 86,400,000 milliseconds per day lands on the wrong calendar date across a daylight-saving change, which is why some calculators quietly disagree with each other twice a year. We add whole calendar days to the (year, month, day) components instead, so a clock change never moves the answer.
Month and year arithmetic clamps, it does not overflow
Adding one month to 31 January gives 28 February (29 February in a leap year), not 3 March. This is the convention every legal and accounting definition of "one month" uses, and it is what most people mean. A calculator that rolls the overflow into the next month will tell you a contract signed on 31 January with a one-month term expires on 3 March. Ours will not.
The same clamping applies to years: 29 February plus one year is 28 February of the following year.
Worked example — a net-30 invoice
An invoice dated Monday 14 September 2026 on net-30 terms is due 14 October 2026, a Wednesday. Counted in business days instead, thirty working days from 14 September is Tuesday 27 October 2026, because the count steps over twelve weekend days and Columbus Day on Monday 12 October — forty-three calendar days in all. The two answers are thirteen days apart, and that gap is exactly why the wording on an invoice matters. "Net 30" almost always means calendar days; "within 30 working days" is a materially longer term.
Worked example — a 45-day deadline
A 45-day identification window opening on 1 May 2027 closes on 15 June 2027. Day 1 is 2 May: the start date itself is not counted, which is the standard convention for a deadline expressed in days and the one we use throughout the site. If your contract counts the start day, subtract one from the result.
Where calculators disagree
- Does the start date count? We do not count it. Day 1 is the day after the start date.
- Month-end overflow. We clamp to the last valid day of the target month.
- Daylight saving. Calendar-day arithmetic is unaffected by clock changes. Hour-based arithmetic is; see the time calculator.
- Which holidays. Business-day mode uses the eleven US federal holidays plus their observed dates. See where our holiday dates come from.
Our full set of conventions is written out on the methodology page.
Worked example — a 90-day probation period
A probation period starting Monday 2 March 2026 and running 90 days ends on Sunday 31 May 2026. That is a weekend, which is the first thing worth noticing: a review scheduled for the end of a calendar-day period regularly lands on a day nobody is at work. The span contains twenty-six weekend days and Memorial Day on 25 May, so in working days it is only 63. Ninety business days from the same start is Thursday 9 July 2026 — five and a half weeks later than the calendar-day answer. If a contract says "90 days" and a handbook says "90 working days", they are describing two different dates and one of them is wrong.
The intervals people actually count, from 18 August 2026
Thirty days lands on Thursday 17 September 2026 (ISO week 38, day 260 of the year). Sixty days lands on Saturday 17 October. Ninety days lands on Monday 16 November. One hundred and eighty days lands on Sunday 14 February 2027, and a full 365 days lands on Wednesday 18 August 2027 — the same date, because 2027 is not a leap year. Add 365 days to a date in a span containing 29 February and you land a day early instead; that is the whole reason "a year" and "365 days" are different instructions. Each of those intervals has its own page: 30 days from today, 60 days, 90 days, 180 days and 365 days, each recomputed every morning.
The month-end trap, with the dates
Clamping is easy to state and easy to get wrong in practice, so here it is with real years. 31 January 2026 plus one month is 28 February 2026. 31 January 2028 plus one month is 29 February 2028, because 2028 is a leap year. And 29 February 2028 plus one year is 28 February 2029. The consequence that catches people: the operation does not reverse. 28 February 2026 minus one month is 28 January 2026, not 31 January. If you need a monthly series that always lands on the last day of the month, do not generate it by repeatedly adding one month to the previous result — take the last day of each month directly.
Counting backwards
Enter a negative number, or switch to subtract, and the same conventions apply in reverse: the start date is still not counted, and month arithmetic still clamps. "30 days before 17 September 2026" is 18 August 2026, which is the mirror of the example above and a useful check — a calculator whose forward and backward answers do not round-trip on calendar days has a bug. Ours round-trips on days and weeks always, and on months and years except where clamping has thrown away information, which it says on the result.
Which counting convention your document uses
The site's convention is that the start date is not counted: day 1 is the day after. That is the ordinary reading of a deadline expressed in days, but it is not universal, and the exceptions are exactly the documents where being wrong is expensive. Federal court filing rules, some state statutes and many insurance policies count the day of the triggering event as day 0 or day 1 by explicit definition — and some roll a deadline forward when it lands on a weekend or a holiday, which is a second rule on top of the first. Read the definition clause before trusting any calculator, including this one. Where a rule counts the start day, subtract one from the answer here.