Add or Subtract Days Calculator
Add or subtract days, weeks, months or years from any date.
Results update as you type.
About this calculator
An add or subtract days calculator shifts a start date by a number of days — and optionally weeks, months and years — and shows the resulting calendar date, which day of the week it falls on, its day-of-year number and the net shift in days. It handles month lengths and leap years automatically, so "3 months from" the 30th or the 31st lands on a real date rather than an impossible one like 31 February.
The way it works is simple: pick a start date, choose Add or Subtract, and enter how many days, weeks, months or years to move. Years and months are applied first against the calendar, then the weeks and days are counted off one by one, so the answer respects the fact that months are 28–31 days long and that leap years have an extra 29 February. The result is a single shifted date together with its weekday, its ISO form (YYYY-MM-DD) and the total net movement in days.
For a worked example, start on Friday 15 August 2025 and add 90 days: the tool returns Thursday 13 November 2025, day 317 of the year, a net shift of +90 days. People reach for this when counting a warranty or return window, a 30/60/90-day notice period, a due date 40 weeks from conception, a court or contract deadline, or simply "what date is 100 days from now".
Frequently asked questions
How are months added to a date?
Months are added by calendar, not by a fixed number of days. Adding one month to 15 January gives 15 February. When the target month is shorter, the date rolls into the following month — 31 January plus one month lands in early March.
Does subtracting cross into the previous year?
Yes. Subtracting more days, weeks or months than remain in the current year simply walks back across the year boundary, correctly accounting for the different length of each month and any leap day.
Can I combine days, weeks, months and years?
Yes. All four fields are applied together in the chosen direction. Years and months are applied first, then the weeks and days, so the result is a single shifted date.
What date is 90 days from a given date?
Enter the start date, leave the operation on Add, and put 90 in the Days field. The calculator counts 90 calendar days forward — for example 90 days after 15 August 2025 is 13 November 2025 — and also names the weekday it lands on.
Does it account for leap years?
Yes. Adding or subtracting across 29 February is handled automatically, so a one-year shift from 1 March 2024 correctly lands on 1 March 2025 even though 2024 contained the extra leap day.
How is this different from the date difference calculator?
This tool takes one date and moves it by a length of time to find a new date. The date difference calculator takes two dates you already know and measures the span between them.
Does it count the start date itself?
No. The start date is the anchor, not day one. Adding one day to Monday gives Tuesday, so the number you enter is the size of the jump, not a position in a list of days.
API — use this calculator from code
Call this calculator as a free JSON endpoint — no key required. Send the field values below as query parameters or JSON. Read the full API docs →
Endpoint
GET https://calculator.free/api/v1/add-subtract-days/curl
curl "https://calculator.free/api/v1/add-subtract-days/?operation=add&days=30"JavaScript fetch()
const r = await fetch(
"https://calculator.free/api/v1/add-subtract-days/?" + new URLSearchParams({
"operation": "add",
"days": "30"
}));
const data = await r.json();
console.log(data.results);Results are estimates for general guidance only, not financial, medical or tax advice.