Date Add Subtract Calculator
Add or subtract days, weeks, months or years to any date, including business days, and see the resulting weekday. Free, instant and precise.
Use YYYY-MM-DD.
New date
2026-09-09
Day of week
Wednesday
Days changed
30 days
Added
2026-08-10 is a Monday. Adding 30 days gives 2026-09-09 — a Wednesday.
How it works
Date ± n units. Months/years clamp the day to the last valid day of the target month.
Example: 10 Aug 2026 + 30 days = 9 Sep 2026 (Tuesday).
Last updated: May 2026
How this calculator is verified
Checked by True Calculator automated test suite on
- Formula verified against a published worked example in the automated test suite
- Edge cases (zero, negative, boundary and unit-mismatch inputs) covered by unit tests
The full verification method is on our how we verify page. Found an error? Tell us and we will re-check it.
When to Use This Calculator
Date addition and subtraction come up in everyday planning: adding 90 days to find a warranty expiry, subtracting months to check a subscription renewal, or counting forward to a project deadline. The tool is the fastest way to turn a delivery date, exam date or bill due date into a calendar date from a starting point, and calendar months stay exact — adding one month to 31 January lands on 28 or 29 February, never on a fixed 30-day step. Business-day mode skips weekends for contracts and delivery promises. It also answers reverse questions, like which date falls 6 weeks before a wedding or 45 days before an event, and replaces both mental arithmetic and date-based spreadsheet formulas.
How to Use This Calculator
- Step 1: Enter the starting date in the date field, using the YYYY-MM-DD format.
- Step 2: Choose Add or Subtract and select the day type — calendar days, or business days which skip weekends.
- Step 3: Enter the amount and pick the unit: days, weeks, months or years (calendar mode only).
- Step 4: Read the new date with its weekday, and the total number of days changed.
Worked Example
Adding 90 days to 15 August 2026 gives 13 November 2026, a Friday — the exact math used for visa windows and delivery deadlines. Subtracting 30 days from 10 August 2026 lands on 11 July 2026. Calendar months follow real month lengths: one month after 31 January 2024 is 29 February 2024, because leap February has 29 days. In business-day mode, adding 10 business days to Monday 10 August 2026 lands on 24 August 2026, skipping the two weekends in between. The weekday shown with each result makes it easy to spot weekend clashes.
Tips and Common Mistakes
- •Tip 1: Add days rather than months when the exact span matters, because month arithmetic depends on the month's length — 90 days is unambiguous, while 3 months is not.
- •Tip 2: Use business days for contracts and delivery promises that exclude weekends, and calendar days for everything else.
- •Tip 3: Adding one month to the 29th, 30th or 31st clamps to the last day of the target month — 31 January plus one month is 28 February in a common year.
- ✗Mistake 1: Expecting add 1 month to always keep the same day number — adding a month to 31 January gives 28 or 29 February, never a March date.
- ✗Mistake 2: Forgetting to switch between add and subtract when working backwards, which silently produces a date on the wrong side of the starting point.
Expert Note
Adding calendar months to end-of-month dates produces clamping: 31 January plus 1 month is 28 February (or 29 in a leap year). For visa windows and delivery deadlines, use day-based arithmetic (90 days) rather than month-based (3 months) to avoid ambiguity.
Sources and References
- •ISO 8601:2019 — Date and time format representation
- •Gregorian calendar month lengths — US Naval Observatory
Official sources are linked so you can confirm the current rate yourself. Check the linked page for the latest notification before relying on these figures.
Frequently Asked Questions
How do I add 90 days to a date?
The calculator adds 90 calendar days to the date you enter. Adding months uses actual calendar months, clamping the day if needed — January 31 plus one month is February 28 (or 29 in a leap year).
What date is 6 months from today?
Enter today's date with 6 months added. The result is the same day in the month six months later, adjusted for shorter months.
Does the calculator handle leap years?
Yes. February 29 exists in leap years, and adding a year to Feb 29 clamps to Feb 28 in non-leap years.
Why does adding a month not equal adding 30 days?
Calendar months have 28 to 31 days, so adding a month keeps the same day number. Adding 30 days instead lands on a different date — choose the unit that matches the calendar you need.
What is date addition?
Date addition means adding a number of days, weeks, months or years to a date to find a new date — for example, adding 90 days to today to find a deadline. This calculator does date addition and subtraction in one step, keeping calendar months exact.
Compare these tools
- Age Calculator — Find your exact age in years, months and days from your date of birth
- Date Difference Calculator — Count the exact days, weeks, months, years and business days between two dates, with hours and minutes too
- Years Calculator — Calculate whole and fractional years between two dates, plus months, days and total days
You might also need
Related calculators from other categories
Math
Basic Calculator
Add, subtract, multiply and divide any two numbers instantly. A simple free calculator for everyday sums and quick checks.
Health & Fitness
Conception Calculator
Estimate when conception happened and your fertile window from your cycle or due date. Understand your pregnancy timeline.
Health & Fitness
Due Date Calculator
Calculate your estimated due date, gestational age and trimester from your LMP — a free pregnancy due date calculator for Indian mothers.
Time
Time Calculator
Add or subtract hours and minutes from any time of day. Handles midnight wrap-around and reports the day shift.