Date calculator: add or subtract days, months and years

To find a date in the future or past, enter a start date and the number of days, weeks, months or years to add or subtract — the answer appears instantly with its weekday. Two rules decide most "off by one" arguments, and this calculator makes both visible: whether the start date counts as day 1, and what happens when you add a month to the 31st. Switch to Days between dates to count the gap between any two dates.

How the calculator works

Every date is converted to a whole-day number and the arithmetic is done on those numbers, so daylight-saving switches and time zones can never push a result onto the wrong day — a surprisingly common bug in calculators that add hours behind the scenes. When you combine units, they are applied from largest to smallest: years and months first, then weeks, then days. That order matters. Starting from January 31, "1 month and 1 day" gives March 1 (January 31 → February 28 → March 1), whereas doing the day first would give March 1 in a different way and can differ in leap years.

Months are added on the calendar, not as 30-day blocks: April 15 plus one month is May 15. If the target month is shorter than the starting day-of-month, the date is moved back to that month's last day and a note explains it. The full reasoning, with how Excel, Python, JavaScript and contracts each handle it, is in adding months to a date: the end-of-month rule.

"Day 0" or "day 1"? The counting rule

Ordinary date arithmetic treats the start date as day 0: 10 days after the 1st is the 11th. Many real rules count the start date as day 1 instead — a 90-day visa that starts on the day you arrive, a prescription "for 7 days" starting today, a 30-day return window that includes the purchase date. Under that rule, the 90th day is one day earlier than "start + 90". Tick Start date counts as day 1 and the calculator shows both. If you are unsure which applies to you, inclusive vs exclusive day counting walks through visas, notice periods, leave and deadlines.

Worked examples

Business days

With Business days ticked, the Days figure skips Saturdays and Sundays: one business day after a Friday is the following Monday. Weeks, months and years are still applied as calendar periods. Public holidays are not removed, because they differ by country, state and even employer; check your local calendar and add a day for each holiday in the range. The Days between dates tab also reports the number of weekdays in a period, which is handy for estimating working days in a project or a notice period.

Common uses

• Travel: the last day of a 30-, 60- or 90-day stay, passport validity windows, and how far ahead a booking opens.
• Work and legal: notice periods, probation end dates, invoice due dates (net 30, net 60), and contract renewal dates.
• Health and family: medication courses, follow-up appointments, and counting the days until an event.
• Finance: the number of days interest accrues between two dates, or when a promotional rate ends.

For anything legally binding, check the exact wording of the rule: whether the first day counts, whether weekends or holidays extend a deadline, and which time zone applies are all set by the rule itself, not by arithmetic.

Frequently asked questions

What date is 90 days from today?

Pick today's date (it is filled in automatically), type 90 in the Days box and the tool shows the date straight away, with the weekday. If the start date itself counts as day 1, as it does for many visa stays, tick 'Start date counts as day 1' and the tool also shows the 90th day, which is one day earlier.

What is January 31 plus one month?

February 28, or February 29 in a leap year. February has no 31st, so the result is moved back to the last day of the month. Excel's EDATE, most programming libraries and most contracts follow this end-of-month rule, and the tool tells you whenever it has applied it.

How do I count the days between two dates?

Open the 'Days between dates' tab and enter both dates. The main figure is end minus start, the standard count used by spreadsheets. The table also shows the count with both dates included, the years-months-days breakdown, and the number of weekdays.

Does the calculator count weekends and holidays?

Calendar days include every day. Tick 'Business days' to skip Saturdays and Sundays when adding or subtracting days. Public holidays differ by country and region, so they are not removed automatically: add one working day for each holiday that falls in the range.

Why does adding 1 month and then 30 days give different dates?

A month is not a fixed number of days. Adding one month keeps the day-of-month (March 15 becomes April 15), whereas adding 30 days moves exactly 30 days, which lands on a different date in 31-day months and in February. Use months when the rule says 'one month' and days when it says '30 days'.

Is my data sent anywhere?

No. All calculations run in JavaScript inside your browser. The dates you type are not uploaded or stored.