Date Calculator
Compare two dates or shift one by a calendar or business-day amount, with US and Korean public holidays built in.
- Fewer clicks, keyboard friendly
How to use
- Pick “Difference” to compare two dates, or “Add or subtract” to shift one date.
- In Difference, set a start and end date to see total days, weeks, and the calendar years/months/days between them.
- Turn on a holiday calendar (United States or South Korea) to also see business days with that country’s public holidays excluded.
- In Add or subtract, set a base date and direction, then enter years/months/weeks/days — or switch to a business-day count.
- Copy the result with one click; your inputs are saved only in this browser so they are there next time.
Why business-day counts need a holiday calendar, not just weekends
Excluding Saturdays and Sundays gets you close to a working-day count, but it still overcounts by one for every public holiday that falls on a weekday. This calculator computes the current 11 US federal holidays directly from their official rules — fixed dates like Independence Day (July 4) and Christmas (December 25) shift to the nearest weekday when they land on a Saturday or Sunday, and floating dates like Memorial Day (last Monday of May) and Thanksgiving (fourth Thursday of November) are derived for each year. Juneteenth is included from 2021, when it became a federal holiday. South Korean public holidays follow the lunar calendar for Seollal and Chuseok, and the government decides substitute holidays (대체공휴일) year by year, so this calculator uses a verified table covering 2024–2027; outside that range it still excludes weekends but shows a note that holidays were not subtracted.
How the years/months/days span is calculated
A plain day count (like 30 days) is exact but not how people talk about durations, so this calculator also walks whole calendar months from the earlier date and keeps the leftover days. From January 31 to March 1 of the same year, for example, adding one whole month to January 31 lands on February 28 (or 29 in a leap year) because February has no 31st, and the single remaining day to March 1 is counted separately — so the result reads as 1 month, 1 day rather than a confusing "1 month, 29 days." The same clamping applies to Add or subtract: January 31 plus one month becomes February 28 in a non-leap year.
What “include the start date” changes
By default, the gap from October 1 to October 3 is counted as 2 days — the nights between them. Turning on "include the start date" counts both endpoints, so the same dates read as 3 days, matching how people describe a 3-day trip that starts on the 1st and ends on the 3rd. This toggle affects both the total-day count and the business-day count the same way, so the two numbers stay consistent with each other.
Frequently asked questions
How many days are there between two dates?
Enter both dates in Difference mode. The Total days stat is the exact calendar-day gap; turn on "include the start date" if you want both endpoints counted instead of just the nights between them.
Does this tool calculate business days, including holidays?
Yes. The Business days stat always excludes Saturdays and Sundays, and excludes United States or South Korean public holidays too once you select that country from the holiday calendar.
How do I add 90 days, or 3 months, to a date?
Switch to "Add or subtract," pick a base date and "Add," then type 90 into the Days field (or 3 into Months). The resulting date and its weekday appear immediately, with no calculate button to press.
Why does January 31 plus one month show February 28?
February has no 31st day, so the result is clamped to the last day of February — the 28th in a common year or the 29th in a leap year — which matches how most calendar software handles month-end overflow.
Which countries’ public holidays are supported?
United States federal holidays are computed from their official rules, with Juneteenth included from 2021 when it became a federal holiday. South Korean public holidays, including lunar-calendar and substitute holidays, are supported for 2024–2027; a notice appears if a chosen date falls outside that range.
Is any date I enter sent to a server?
No. Every calculation runs in your browser with plain JavaScript, and your last inputs are saved only in this browser’s local storage, not on a server or in any account.
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Updated 2026-10-14