Date and time calculators

How date counting works

Every calculator on this site follows the same few rules, and most disagreements between date calculators come from one of them. Knowing them makes it easy to check any answer.

1. The start day is day 0

Counting from a date means starting the next day: 1 day from Monday is Tuesday, 7 days from Monday is the next Monday. Between two dates, the count is the number of days you step over, so Monday to Wednesday is 2 days. This “exclusive” count is what deadlines, invoices (“net 30”) and most laws use. The US federal court rules spell it out: exclude the day of the event, count every day, include the last day (FRCP 6(a)(1)). Counting the start day as well (“inclusive”) adds one day, and is right for things like the number of days of a trip or a leave period, where both the first and the last day are used.

2. Days, weeks, months and years are different units

A day and a week are fixed lengths, so adding them is simple arithmetic. Months and years are calendar units: one month from January 31 is the end of February, and one year from February 29 is February 28 in a common year. The date calculators add months by keeping the day of the month and using the month’s last day when that day does not exist, the same rule as spreadsheet functions such as EDATE and most programming libraries. Thirty days and one month therefore end up to two days apart (a month is 28 to 31 days), and 365 days and one year differ by one day when a 29 February is inside.

3. Leap years are part of the calendar

A year divisible by 4 is a leap year, except century years not divisible by 400: 2000 was a leap year, 2100 will not be (US Naval Observatory). So the Gregorian calendar has 97 leap days every 400 years, an average year of 365.2425 days, and it repeats exactly every 400 years, weekdays included. The calculators count with a running day number for every date, the Julian Day Number, so leap days are handled without special cases.

4. Business days skip weekends and holidays

Business-day counts skip Saturdays and Sundays (or the local weekend, such as Friday and Saturday in Saudi Arabia or Egypt) and, if you choose a country, its public holidays. Holiday lists come from official government sources for 2026–2028, including the weekday a holiday moves to when it falls on a weekend. Count business days from the next working day: 1 business day from a Friday is Monday.

5. “Today” is your local date

Dates depend on the time zone: when it is evening in New York it is already tomorrow in Tokyo. Every page here uses your device’s time zone and moves on at your midnight, so a page left open overnight shows the new date without reloading. Calculations in hours, such as “8 hours from now”, add real elapsed time, so when a daylight saving change happens in between, the clock time comes out one hour different from simple clock arithmetic; the time calculator shows when that happens.

Which calculator to use

• A date a number of days, weeks or months away: days from today, weeks from today or the date calculator for any start date.
• How long between two dates: days between two dates; working days only: the business days calculator.
• How long until something: days until a date, or the timer for minutes and hours.
• How old someone is: the age calculator. What weekday a date was: what day is a date.