Date Calculator & Business Days

About Date Calculator & Business Days

This date calculator has five tabs: a D-Day countdown, the difference between two dates (total days, years/months/days, weekday and weekend counts), adding or subtracting days, weeks, months or years, and two business-day modes: working days in a range, and the date N business days before or after another.

Date math rarely goes wrong on the arithmetic. It goes wrong on conventions: whether the first day counts, what “one month after January 31” means, and which holidays to skip. The modes don't all follow the same convention, so this page walks through them one by one.

Does the first day count? It depends on the mode

D-Day and date difference are plain subtraction: later date minus earlier date. From September 22 to September 23 is 1 day. The weekday/weekend split in the difference mode also counts from the day after the start through the end date. The working-days-in-range mode is different: it counts both endpoints. September 22–23 is 2 working days there.

Subtraction matches how most legal deadlines work. U.S. Federal Rule of Civil Procedure 6(a) says to exclude the day of the event that triggers the period, count every day including weekends and holidays, and include the last day, unless the last day is a Saturday, Sunday or legal holiday, in which case the period runs to the next day that isn't. Korea's Civil Act (Article 157) and Japan's Civil Code (Article 140) likewise leave out the first day. To get a “within 14 days of service” deadline, add 14 days to the service date in the add/subtract mode. The tool won't roll a weekend deadline forward, so check the weekday of the result yourself.

Anniversaries and “day counts” often use the opposite convention, where the start date is day 1. If you want the 100th day of something that began today, add 99 days, not 100.

What is one month after January 31?

When adding months or years lands on a day that doesn't exist, the tool clamps to the last day of that month. January 31, 2026 plus one month is February 28; in 2024 it's February 29. February 29, 2024 plus one year is February 28, 2025. Spreadsheet functions like EDATE behave the same way, and so do the civil codes of Korea, Japan and Spain when a period measured in months ends in a shorter month.

The side effect is that month arithmetic isn't associative. January 31 plus two months is March 31, but January 31 plus one month, plus another month, is March 28, because the first step lost three days to February. For recurring dates like end-of-month billing, always add from the original date.

The years/months/days breakdown in the difference mode uses the same rule. January 31 to March 1 is 29 days in 2026 and 30 days in 2024, but both show as “1 month 1 day,” because the one-month mark is pinned to the end of February.

Which holidays the business-day modes know

The holiday data covers South Korea only, for 2023 through 2027. Korea's biggest holidays, Seollal and Chuseok, follow the lunar calendar, so the tool stores each year's confirmed dates instead of computing them. Substitute holidays, election days (the 2024 general election, the June 3, 2025 presidential election and the June 3, 2026 local elections) and one-off holidays such as January 27, 2025 are included, as are Constitution Day (July 17) and Labor Day (May 1), both public holidays again from 2026.

For anywhere else, switch holiday exclusion off, which leaves a plain Monday–Friday count, and subtract your own holidays. In the U.S. that means the eleven federal holidays in 5 U.S.C. § 6103, including Juneteenth (June 19) since 2021, plus any state or company days off. Outside 2023–2027 the tool also falls back to weekends only and names the uncovered years in the result.

The half-day options subtract 0.5 when the start or end date is itself a working day, which helps when you're counting leave taken in half days.

Worked examples

How to use it

1. Pick a mode from the tabs at the top.
2. Enter the dates; the difference and range modes sort them automatically if you enter them backwards.
3. In the business-day modes, decide whether to exclude (Korean) holidays and whether the ends are half days.

Frequently asked questions