Date Calculator

Adding or subtracting days and weeks

Days are the simplest unit. The date calculator steps forward or back the exact number of calendar days, crossing month and year boundaries as needed and including February 29 in leap years. Weeks are converted to days first, so 6 weeks is the same as 42 days, and the result always falls on the same day of the week as the start date.

The start date counts as day 0. Adding 1 day gives tomorrow, and subtracting 1 day gives yesterday. This matches how most notice periods and “within X days” deadlines are written, but check the wording if a missed deadline would be costly. Some rules count the first day as day 1.

Note that 52 weeks is 364 days, one day short of a common year and two short of a leap year. A date 52 weeks later falls on the same weekday but one or two calendar days earlier.

How to add or subtract days by hand

To add a number of days without the calculator, use up the remaining days of each month until the total is spent. Take 150 days after Sunday, October 4, 2026.

1. October has 31 days, so reaching October 31 uses 27 days.
2. November (30), December (31) and January (31) bring the running total to 27 + 30 + 31 + 31 = 119 days, on January 31, 2027.
3. February 2027 has 28 days, so February 28 is day 147.
4. The remaining 3 days land on Wednesday, March 3, 2027.

Subtracting days from a date

Subtracting works the same way in reverse. Thirty days before Sunday, March 15, 2026 is Friday, February 13, 2026: 15 days takes you back to February 28, and 15 more take you to February 13.

Adding or subtracting months and years

Months are added on the calendar, keeping the same day of the month: March 15 plus 2 months is May 15. Years are added as 12 months each.

Because months have different lengths, some dates don’t exist in the target month. When that happens, the calculator uses the last day of the target month. For example, August 31 plus 1 month is September 30, because September has only 30 days. A note appears under the result whenever this adjustment is made.

This means month arithmetic doesn’t always reverse cleanly. August 31 plus 1 month is September 30, but September 30 minus 1 month is August 30. When the exact day matters, as with rent, subscriptions or loan payments, check how your agreement handles short months.

Month-end examples

Each step is measured from the original date, not chained. January 31 plus 2 months is March 31, even though January 31 plus 1 month is February 28.

Days in each month

Dates from Sunday, October 4, 2026

For reference, here are some common offsets from Sunday, October 4, 2026, the date this page was last updated. Enter today’s date or any other start date in the calculator for your own results.

Notice that 90 days and 3 months are not the same: here they differ by two days.

Leap years

A leap year has 366 days, with the extra day on February 29. Under the Gregorian calendar, a year is a leap year if it is divisible by 4, except for century years, which must also be divisible by 400. So 2024 and 2028 are leap years, 2000 was one, and 2100 will not be. The date calculator applies this rule automatically. For the history and reasoning behind the rule, see how leap years work.

Where date arithmetic is used

• Notice periods and contracts. A 30-day notice period or a 6-month probation is a date calculation. Use days or months to match the wording of the agreement.
• Deadlines and follow-ups. Payment terms such as “net 30”, response windows and warranty periods are usually counted in days from an invoice, delivery or purchase date.
• Recurring dates. Monthly bills, subscription renewals and loan payments are month additions, which is where month-end clamping matters. For payment schedules themselves, the loan calculator shows each payment.
• Counting back. Subtracting is useful for working out when to start: 12 weeks before an exam or event, or the latest date to send something that must arrive within 10 days.

Tips and common mistakes

• Choosing the wrong unit. “One month” and “30 days” can give different dates. Use whichever the rule or agreement actually says.
• Counting the start date as day 1. The calculator treats it as day 0. If your rule counts the first day, enter one day fewer.
• Forgetting weekends and holidays. Many deadlines that fall on a weekend or public holiday move to the next working day. The calculator does not make that adjustment.
• Ignoring time zones. Dates are treated as calendar dates with no time of day. If a deadline is set in another time zone, check which local date applies.

Glossary

• Day 0. The start date itself. The first day after it is day 1.
• Month-end clamping. Moving a result to the last day of the month when the same day number doesn’t exist there.
• Leap year. A 366-day year with a February 29.
• Calendar day. Any day of the week, including weekends and holidays, as opposed to a business day.

To count the days between two dates you already know, use the days between dates calculator. To find the date someone reaches a particular age, add years to their date of birth here or use the age calculator.