What happens if I add a month to 31 January?

The result is 28 February (or 29 in a leap year). Overflowing days clamp to the last valid day of the target month, matching the behavior of the browser Date API.

Are holidays excluded from the business days count?

No. Business days counts every Monday through Friday in the range and excludes Saturdays and Sundays. Public holidays are not excluded because they vary by country and region.

Can I measure a duration where the end date is before the start date?

Yes. The calculator supports negative durations — if To is earlier than From, the values come out negative.

In what order are years, months, weeks, and days applied?

Years first, then months, then weeks (as seven days), then days. The order matters because month lengths vary, so adding 1 month and 30 days is not the same as adding 30 days and 1 month.

Date Calculator

Add or subtract days, weeks, months, and years from a date — or find the duration between two dates.

Add or subtract from a date

Start dateYYYY-MM-DD

Operation

Years

Months

Weeks

Days

Result

Wednesday, September 9, 2026

2026-09-09

How to use the Date Calculator

Use Add or subtract mode to shift a date by a number of calendar units, or Duration between mode to measure the span between two dates.

Choose a mode at the top: Add or subtract to shift a date by years/months/weeks/days, or Duration between to measure the span between two dates.
In Add or subtract mode, pick a start date, choose Add or Subtract, and fill in any combination of years, months, weeks, and days. The result updates instantly.
In Duration between mode, pick the From and To dates. You will see a calendar breakdown (years, months, days), total days, weeks with remainder, and business days excluding weekends.
Read the result — the full weekday and calendar date are shown alongside the ISO 8601 representation so you can copy either.
Swap to the other mode any time without losing your input in the current mode; each mode keeps its own state.

Frequently asked questions

What happens if I add a month to 31 January?: The result is 28 February (or 29 in a leap year). Overflowing days clamp to the last valid day of the target month, matching the behavior of the browser Date API.
Are holidays excluded from the business days count?: No. Business days counts every Monday through Friday in the range and excludes Saturdays and Sundays. Public holidays are not excluded because they vary by country and region.
Can I measure a duration where the end date is before the start date?: Yes. The calculator supports negative durations — if To is earlier than From, the values come out negative.
In what order are years, months, weeks, and days applied?: Years first, then months, then weeks (as seven days), then days. The order matters because month lengths vary, so adding 1 month and 30 days is not the same as adding 30 days and 1 month.