Date Calculator

• Dates are calendar dates on your device. Times of day are not used, so daylight saving time changes do not add or remove an hour.
• Leap years are followed. In a year with February 29, that day counts like any other.
• Month-end rule: when adding months or years gives a day the month does not have, the result moves to the last day of that month.
• The number of days between two dates is the end date minus the start date. Count the start date as day 1 adds 1 to that.
• Weeks and days is the number of days divided by 7, with the remainder as days.
• Months and days first finds the largest number of whole months you can add to the start date without passing the end date, then adds the days left over.
• Days and weeks are added as a number of days. Months and years move to the same day of the month in the calendar, then the month-end rule applies.
• If the end date is before the start date, the two dates are counted the other way round and the result is marked “earlier”.
• You can enter dates from January 1, 1900 to December 31, 2100. The number to add or subtract can be from 1 to 100,000.

How this compares with deadlines in contracts and law

Many legal and contractual rules count a period of days from the day after the event that starts it, and include the last day. The Add or subtract mode counts the same way, because it starts counting the day after the base date. Add 10 days to November 1 and you get November 11, which is the 10th day when November 2 is day 1. In Between two dates, leaving Count the start date as day 1 off gives the same kind of count.

Some rules also move a deadline that falls on a weekend or public holiday to the next working day. Whether that applies depends on the country, the court or agency, and the contract. This calculator does not know holidays, so check the day of the week in the result against a calendar. When a contract states its first and last day, follow the contract.

Every result below was produced by this calculator.

Between two dates

A fall semester that runs from September 8 to December 18 is 101 days if you subtract the first day from the last. If both the first and the last day of classes count, it is 102 days, or a little over 14 and a half weeks.

Add or subtract

A purchase made on November 11, 2026 with a 90-day return window can be returned until February 9, 2027. If an application is due on December 31, the date 45 days earlier is November 16. Six months after August 31 would be February 31, which does not exist, so the result moves to February 28, a Sunday.

Age and time since a date

438 months is 36 years and 6 months, so someone born on May 10, 1990 is 36 years old on November 11, 2026, and their next birthday is about six months away.

• Counting the start date: the same dates give one more day when the start date counts. That is why the first example gives 364 days or 365 days.
• One month after the 31st: February has no 31st, so January 31 + 1 month is February 28 (February 29 in a leap year). If you add one month to January 31, 2026 and then subtract one month, you get January 28, 2026, not January 31.
• One year after February 29: February 29, 2024 + 1 year is February 28, 2025.
• A month is not a fixed number of days. January 31 to February 28 is 28 days, but it shows as “1 month 0 days”, because January 31 + 1 month is February 28. To turn a period given in months into days, do not multiply by 30. Add the months here and count the days between the two dates.
• Weekends and holidays are not skipped. The calculator counts calendar days only. A last day that falls on a Saturday, a Sunday or a public holiday is not moved.
• End date before the start date: instead of a negative number, the result says how many days earlier.
• Dates outside the supported range are not calculated, and you see why. If a result would be after December 31, 2100, you are asked to use a smaller number.

When should I turn on “Count the start date as day 1”?

Turn it on when the first day counts as a full day, for example when you want to know which day of a trip, a course or a treatment today is. Leave it off when you only want to know how far apart two dates are.

Can it count only weekdays or business days?

No. It counts every day on the calendar. Public holidays differ by country and region, so they are not built in.

Does it use my time zone?

It uses the date on your device. Times of day are not part of the calculation, so the result does not change with daylight saving time. If you and someone in another time zone open the page around midnight, today’s date can differ by one day.

How do I find when a contract or lease ends?

Use Add or subtract: enter the start date and add the length of the contract in months or years. If the period is counted from the day after the start date, that result is the last day. Many leases instead start at the beginning of the start date, so a one-year lease that starts on November 15, 2026 ends on November 14, 2027, one day before the result. If the result says the last day of the month was used, take that date as it is. If the contract states an end date, that date wins.

Can I work out someone’s age?

Yes. In Between two dates, enter the date of birth and today’s date, then divide the months in Months and days by 12. 438 months 1 day is 36 years, 6 months and 1 day, so the person is 36.

What is the date 100 days from today?

Use Add or subtract and add 100 days. 100 days after November 11, 2026 is February 19, 2027. If you want a milestone where the start day itself is day 1, such as a 100-day anniversary, the D-Day Calculator counts it for you.