Date calculator

Pick a start date, choose add or subtract, and enter an amount in days, weeks, months, or years. Month moves use constrain overflow and years-then-months-then-days order. February 29 defaults to March 1 in non-leap years with a February 28 toggle. Business days skip weekends and custom holidays only. Share links restore mode and values in the URL fragment.

Add or subtract time

How we count

The amount is a magnitude. Direction comes only from Add or Subtract. Negative amounts are rejected so the control and the number cannot disagree.
Month arithmetic uses constrain overflow: January 31 plus one month is February 28 or 29, not March 2 or 3.
When a calculation mixes units internally, we apply years, then months, then days. Reversing that order changes the answer on real inputs.
February 29 landing in a non-leap target year defaults to March 1, with a toggle to February 28. Multi-year jumps go straight to the target year, so 29 February 2024 plus four years is 29 February 2028, a leap year where no anniversary rule applies. UK law treats anniversaries differently from some US practice; we disclose the choice rather than hide it.
Calendar moves never use 86,400,000 millisecond steps, so daylight saving clock changes do not shift the calendar result.

Start date Direction Amount Unit February 29 in non-leap years Business days (days or weeks only)

Custom holidays (optional, one YYYY-MM-DD per line)

Waiting for a start date and amount.

Measured on this page, in this browser

Requests: · from Resource Timing
Script: · KB of a 100 KB budget

Everything here runs in your browser. We never send the start date and amount you enter to ourselves or anywhere else. It stays on your device. Once the page has loaded, you can disconnect and keep working. How we build tools

One page, two directions

Adding ninety days and subtracting ninety days live in one control so a shared link can restore the direction as well as the numbers. That matches the vault ruling: separate URLs for different jobs, one engine underneath.

January 31 plus one month

Enter January 31, add, amount 1, unit months. The result is February 28, or February 29 in a leap year, never March 2 or March 3. That is constrain overflow: the 31st does not exist in February, so the date clamps to the last day of the target month. Subtract the same amount and February 28 minus one month is January 28, not January 31, because the reverse trip is not a round trip. The end-date counting explainer is the sibling question for spans between two fixed dates, which this page does not compute.

Business days, same scope as days between

Business-day mode excludes weekends and any dates you paste yourself. Country holiday calendars are not built in because they need maintenance this repository does not carry. What 5 to 7 business days means is the shipping-window version of that skip. Use it when the phrase is the question; use this page when you already have a start date and an amount.

Questions

Why does month order matter?

January 31 plus thirty days then one month lands on a different calendar date than January 31 plus one month then thirty days. This page applies months before days when both apply.

Need the span between two fixed dates?

Use the days between dates calculator, which reports inclusive or exclusive counts and a full breakdown.

Related tools

Need the span between two dates instead? Use the days between dates calculator. Does the end date count and what 5 to 7 business days means are the matching explainers. More date tools live on the date and time hub, including the time zone meeting planner. Does the end date count is the article that uses this calculator.

Maintained by Aaron Wilson
Published August 19, 2026. Last updated September 10, 2026. Method and formula: how we build tools. Something wrong? Tell us and it goes in the changelog.