Add or Subtract Days Calculator

Pick a start date, choose whether to add or subtract, and type a number of days. The calculator returns the resulting date written out in full — weekday, day, month and year — plus the same date in ISO YYYY-MM-DD format for pasting into spreadsheets or forms. The start date defaults to today, and everything updates as you type.

How the date is worked out

Each day is treated as a fixed block of 86,400,000 milliseconds — twenty-four hours of exactly sixty-second minutes. The calculator converts the start date into that internal count, anchors it to noon UTC, then shifts it forward or backward by the requested number of whole days:

result = start + sign × days × 86,400,000

Both start and result are millisecond counts, so the number of days has to be multiplied by the length of a day before it is added. The sign is +1 when adding and −1 when subtracting. The shifted count is then read back as a UTC date, and the weekday and the ISO string are both taken from that reading. Using the defaults — 22 July 2026, add, 90 days — the count runs 9 days left in July, 31 in August, 30 in September, then 20 into October. That is 9 + 31 + 30 + 20 = 90, landing on 20 October 2026, which is a Tuesday.

Because the arithmetic runs in days rather than month names, leap years and uneven month lengths take care of themselves. Add one day to 28 February 2024 and the count lands on 29 February, since that year is divisible by 4; the same step in 2025 lands on 1 March. Nothing in the formula knows how long February is — it advances the underlying day count and reads back whatever calendar date that count corresponds to. Anchoring to noon rather than midnight is deliberate: it keeps a daylight-saving hour from ever nudging the answer onto the day before or after.

Why 90 days is not three months

The single most common mistake with deadlines is treating a day count and a month count as interchangeable. They are not. Adding three calendar months to 22 July keeps the 22nd and moves the month, giving 22 October — 92 days, because the months in between run long. Adding 90 days ignores month names entirely and just counts, so it stops on 20 October, two days short.

Whenever a rule is written in days, count days. This tool never silently converts one into the other.

A short history of counting days

The calendar the tool counts in descends from Julius Caesar, who in 46 BC replaced the drifting Roman republican calendar with a solar year of 365 days plus a leap day every fourth year. Getting the seasons back into position first required a one-off correction: extra days were inserted into 46 BC to stretch it to 445 days, the longest year on record, known afterwards as the last year of confusion. That Julian scheme, effective from 1 January 45 BC, assumed a year of 365.25 days — roughly eleven minutes too long. Over centuries the small error accumulated, and by the 1500s the calendar had slipped about ten days out of step with the seasons, pushing the spring equinox away from the date the Church used to fix Easter.

Pope Gregory XIII addressed both problems at once. His bull Inter gravissimas, issued on 24 February 1582, deleted ten days so that Thursday 4 October 1582 was followed directly by Friday 15 October, and it tightened the leap-year rule so that century years count as leap years only when divisible by 400. That gives a mean year of 365.2425 days, accurate enough that the residual drift is now roughly one day in 3,000 years. Catholic states adopted the change within a year or two; others held out for generations. Britain and its colonies waited until 1752, by which point the gap had grown to eleven days, so Wednesday 2 September 1752 was followed by Thursday 14 September.

Reconciling events recorded in rival calendars was the problem the scholar Joseph Justus Scaliger set out to solve. In 1583 he proposed the Julian period — which, by his own account, he named after the Julian calendar year it was built on, though it is often wrongly said to honour his father, Julius Caesar Scaliger — a continuous count of days running from a starting point of 1 January 4713 BC in the proleptic Julian calendar. He picked that year because it is where three traditional cycles line up at 1 together: the 28-year solar cycle, the 19-year Metonic lunar cycle and the 15-year indiction used in Roman administration. Their lowest common multiple, 7,980 years, sets the length of the period. By numbering every day in one unbroken sequence, historians and astronomers could measure the interval between two events without wrestling with month lengths, leap days or calendar reforms. Astronomers still use the Julian day, and by convention each one begins at noon rather than midnight so that a single night of observation never straddles two dates. That is the same reason this calculator anchors its arithmetic to noon.

Computers keep time in much the same spirit. Most systems store a moment as the number of fixed-length days or seconds elapsed since an epoch — commonly 1 January 1970 — and derive the calendar date from that number on demand, which is why day arithmetic is fast and exact. The output format YYYY-MM-DD comes from ISO 8601, the international standard first published in 1988, which orders the fields from largest to smallest so that dates sort correctly as plain text and cannot be misread as day-first or month-first.

Deadlines and return windows

Day counting is the everyday version of this math. US retailers commonly run 30-, 60- or 90-day return windows counted from the delivery or purchase date, and warranty and free-trial periods work the same way. Notice periods on tenancies and contracts are usually a fixed number of days too.

Conventions differ by market. Under UK and EU consumer rules a cooling-off period is often expressed as 14 calendar days, and GDPR response deadlines use "one month" rather than 30 days, counted to the matching date in the next month, so a request logged on 31 January runs to the end of 28 February, or 29 February in a leap year, because February has no 31st. Schengen visitors are limited to 90 days within any rolling 180-day span.

US federal courts spell their convention out in Rule 6(a)(1) of the Federal Rules of Civil Procedure: exclude the day of the event that triggers the period, count every day after it including intermediate Saturdays, Sundays and legal holidays, and include the last day — but if that last day is a Saturday, Sunday or legal holiday, the period runs on until the end of the next day that is none of those. Federal tax deadlines roll forward the same way under section 7503 of the Internal Revenue Code. The first two steps are exactly what this calculator does; the roll-forward at the end is the part you apply yourself. Other rules instead count inclusively, treating the trigger day as day one, so check which convention a deadline uses, and whether it runs in calendar days or working days, before you rely on the number.

What the count assumes

A few conventions are worth stating outright. The start date is day zero, so adding one day moves to the next calendar date rather than returning the start. Every calendar day is counted, weekends and public holidays included; if you need only working days, reach for a business-days tool instead. The result is a pure Gregorian date extended backward and forward without regard to the reforms described above, which means dates before 1582 are proleptic and will not match what a contemporary would have written down. And because this is calendar arithmetic rather than clock arithmetic, the time of day and your local time zone play no part in the answer. The day count is capped at 1,000,000 days, about 2,738 years, which is far beyond any real deadline and keeps the result inside the span a calendar date can hold.

Frequently asked questions

What date is 90 days from today?

Set the start to today, keep the direction on add, and leave the count at 90. Starting from 22 July 2026 the answer is Tuesday, 20 October 2026. The calculator counts every calendar day, including weekends and holidays, which is how most return windows and legal deadlines are measured.

Is 90 days the same as three months?

Almost never. Three calendar months from 22 July 2026 is 22 October, which is 92 days away because July and August each have 31 days. Ninety days lands two days earlier, on 20 October. If a policy says days, count days; if it says months, count months.

Does the calculator count the start day itself?

No. Adding 1 day to 22 July gives 23 July, not 22 July, so the start date is day zero. Some legal deadlines are counted inclusively, meaning they treat the start date as day one — in that case subtract one from your figure before entering it.

Does it handle leap years and month lengths correctly?

Yes. Adding 1 day to 28 February 2024 returns 29 February because 2024 is a leap year, while the same step in 2025 returns 1 March. The math works in fixed 24-hour days anchored to noon UTC, so daylight-saving shifts never push a result onto the wrong calendar date.