Date Add and Subtract Calculator

How to use the date add and subtract calculator

Set a start date, choose add or subtract, then fill in whichever of years, months, weeks and days your question uses. The tool applies the whole interval in one shift and returns the resulting date, the weekday it falls on, and the number of calendar days the date actually moved. The example already loaded adds 90 calendar days to 31 July 2026 and returns Thursday 29 October 2026.

Any unit can stay at zero, and weeks are simply seven days each, so entering three weeks and entering twenty-one days give the identical answer. The add and subtract toggle flips the entire interval rather than individual fields, which is what stops the common mistake of adding months while subtracting days.

The order the units are applied in is fixed and it is not an implementation detail: years first, then months, then the week and day part. Calendar arithmetic is not commutative, so a different order genuinely gives a different date.

Start at 29 January 2026, add one month and then one day, and you get Sunday 1 March 2026, because one month lands on 28 February and the extra day rolls into March. Add the day first and then the month and you get Saturday 28 February 2026 instead, a full day apart.

Neither answer is wrong; they answer differently ordered questions. This tool states its order rather than leaving you to discover it, and it always applies the larger units first.

Month arithmetic has that convention stated up front because there is no universally correct answer to it. The tool flags when a clamp has happened rather than hiding it, because a clamp is exactly the case where a contract, a lease or a medication schedule may want a different rule.

The clamp also makes month addition lossy in an interesting way: adding one month twice to 31 January 2026 gives 28 March, while adding two months at once gives 31 March.

What changes in business-day mode?

Business-day mode changes the meaning of the week and day part rather than filtering the result afterwards. Each step moves to the next weekday, skipping Saturday and Sunday, so the answer can never land on a weekend no matter how many days you enter.

Adding one business day to Friday 31 July 2026 gives Monday 3 August, which is three calendar days later, and adding 90 business days to the same start gives Friday 4 December 2026, which is 126 calendar days later. The net calendar days line is there so that gap is visible rather than surprising.

Public holidays are not skipped, because they differ by country, state and employer, so a business-day result that lands on one of yours needs one more step by hand.

The formula, worked line by line

The shift runs in a fixed sequence. Years and months are collapsed into a single month count and applied first, with the day of the month clamped to the length of whatever month the result lands in. Weeks and days are collapsed into a single day count and applied second, either as plain calendar days or as steps over weekdays only.

Subtraction is the same procedure with the sign flipped on both parts, which is why moving forward and then back by the same interval always returns you to the start date.

The day part is computed at UTC midnight, so every step is exactly 86,400,000 milliseconds. Adding days to a local timestamp is where date arithmetic usually goes wrong: an hour lost or gained at a daylight-saving transition is enough to move a rounded result by a whole day.

Civil dates carry no time zone, so the failure mode does not exist here rather than being corrected for. The same reasoning is why the result is a date and a weekday, never a time.

step 1  months total = sign x (years x 12 + months)
        result = start shifted by months total, day clamped to the target month length
step 2  day part = sign x (weeks x 7 + days)
        calendar mode: result = result + day part, every day counted
        business mode: result steps over Mon to Fri only, weekends skipped
net calendar days = result minus start, in whole days, signed

Order matters, and stating it is the only honest option. Starting from 29 January 2026, adding one month and then one day gives Sunday 1 March 2026, while adding one day and then one month gives Saturday 28 February 2026. Subtraction shows the same effect: 31 March 2026 minus one month then minus one day is Friday 27 February 2026, but minus one day then minus one month is Saturday 28 February 2026.

This tool always resolves years and months before days, which matches how contract and notice-period clauses are normally read. If a document of yours specifies a different order, do the two shifts separately and use the tool twice.

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