Add or subtract from a date

What a date calculator is for

A date calculator answers the everyday question "what date is it going to be?" You know a starting point and an offset, and you need the precise calendar date that lands on. The classic cases are a deadline ("net 30 from the invoice date"), a milestone ("90 days after surgery", "a year and a day after the closing"), a renewal ("the subscription bills six months from sign-up"), or simple planning ("three weeks before the wedding"). Doing this in your head is error-prone because months have different lengths, leap years add a day, and the answer often has to skip weekends. This tool removes the guesswork: enter the start date and the offset, and it returns the resulting date together with the day of the week and the ISO week number, so you can drop it straight into a contract, a calendar, or a project plan.

Calendar math versus business-day math

There are two completely different ways to count forward, and choosing the wrong one is the most common mistake people make with dates. Calendar mode counts every single day. It is the right choice when the rule is expressed in real time, such as "within one year" or "30 calendar days", and it lets you combine years, months, weeks, and days in a single calculation. Business-day mode counts only working days. It steps over Saturdays and Sundays, and if you pick a country it also steps over that country's public holidays, which it loads automatically. Use it whenever the rule says "business days" or "working days", which is typical of payment terms, shipping estimates, notice periods, and many legal filing windows. The two methods can produce dates weeks apart over a long span, so always match the mode to the wording of the rule you are applying.

How the month and year arithmetic works

Adding months is surprisingly subtle. What is one month after 31 January? There is no 31 February, so the tool clamps the day to the last valid day of the target month, giving 28 February in a common year and 29 February in a leap year. This clamping behaviour is the same one spreadsheets and the ISO 8601 standard use, and it is why a monthly plan that starts on the 31st renews on the 30th, 28th, or 29th in the shorter months and snaps back to the 31st whenever the month is long enough. Years are applied as a block of twelve months, so the same day-clamping protects 29 February: one year after 29 February 2024 resolves to 28 February 2025. Once the year and month shift is settled, the calculator adds any whole weeks and days as exact 24-hour offsets. Because every date is anchored to UTC midnight, those offsets never drift by an hour across a daylight saving transition, which is a real bug in naive local-time date math.

Worked examples people search for

Most date-math questions fall into a handful of patterns. Here is how each one maps onto the calculator above, so you can reproduce the answer for your own dates.

• "90 days from today" , leave the start date on today, keep the direction on add, stay in calendar mode, and type 90 in the days field. You get the exact date three months out, including its weekday, which is what payment-term and prescription-refill reminders usually need.
• "Net 30 from the invoice date" , set the start date to the invoice date and add 30 days. If your contract says the clock runs in business days rather than calendar days, switch to business-day mode and pick the relevant country so weekends and public holidays are skipped; net-30 in calendar days and net-30 in working days can differ by almost a week.
• "One year and a day" , a common legal and real-estate threshold. Add 1 year and 1 day in calendar mode. Because years are applied as a clamped twelve-month block, a start of 29 February resolves cleanly to 1 March of the following common year rather than throwing an invalid date.
• "180 days before a deadline" , set the start date to the deadline, switch the direction to subtract, and enter 180 days. This is the fast way to find the latest date you can start a process that has a fixed end, such as a visa window or an enrolment cut-off.
• "Six weeks postpartum" or "three weeks before the wedding" , weeks are exact seven-day blocks, so put the count in the weeks field and pick add or subtract. The result lands on the same weekday as the start date, which is often the whole point of counting in weeks.

Sharing and embedding a result

Every calculation is encoded in the page URL, so the Share button copies a link that reopens the tool with your exact start date, direction, offset, and mode already filled in. That is handy for sending a deadline to a colleague or saving a renewal date for later. You can also embed the calculator on your own site with the Embed button, which generates a lightweight iframe snippet that respects your visitor's light or dark theme.

Related calculators

If you already know both ends and want the gap between them, use the days-between tool instead. For a pure working-day count between two dates that also subtracts public holidays, use the working-days calculator, and to work out an exact age or anniversary from a birth date, try the age calculator.

Frequently asked questions