Date
calculator.
Add or subtract time from a date, count business days with your own holiday list, or find exactly how far apart two dates are. Every rule the calculator uses is written out below.
Calendar math,
without surprises.
- Dates use the proleptic Gregorian calendar (ISO 8601), years 0001–9999.
- Adding months or years keeps the day number; if that day does not exist, the result moves to the last day of the month (31 January + 1 month = 28 or 29 February).
- Years and months are applied first, then weeks and days.
- “Days between” counts from the start date to the end date: the start is excluded and the end is included, unless you choose to count both.
- Business days are Monday to Friday. Public holidays are only skipped if you list them.
- Everything is calculated on whole calendar dates, so time zones and daylight saving cannot shift the answer.
How do you add months to a date?
Move the month forward and keep the same day number. 15 March plus one month is 15 April. The tricky case is the end of the month: 31 January plus one month would be “31 February”, which does not exist, so the calculator uses the last day of February instead. This month-end rule matches the date libraries in Java and in JavaScript's Temporal API.
Because of that rule, adding one month and then subtracting one month does not always return to the start date: 31 January → 28 February → 28 January. When the exact number of days matters, add days or weeks instead.
How are the days between two dates counted?
The calculator subtracts the start date from the end date. From 1 March to 4 March is 3 days: you count the 2nd, 3rd and 4th. That is the convention used for durations, ages and deadlines such as “30 days from today”.
If you need to count every date on a calendar, such as the number of days a hotel is booked or the days of an event, choose Include start and end. That adds one day: 1 to 4 March becomes 4 days.
days = end date − start date (default)
days = end date − start date + 1 (count both dates)How are business days counted?
Business days are the weekdays Monday to Friday. When adding business days, the calculator steps forward one date at a time and only counts weekdays that are not in your holiday list. Adding 1 business day to a Friday gives the following Monday.
When counting business days between two dates, the default excludes the start date and includes the end date, just like the calendar-day count. Paste your own holiday dates (for example your country's public holidays, or office closure days) to exclude them. Holidays that fall on a weekend are ignored, because those days were never counted.
Why does a month have no fixed number of days?
Calendar months run from 28 to 31 days, so “3 months” is between 89 and 92 days depending on where you start. The calculator always uses the real calendar rather than an average month, and it shows the exact calendar-day offset next to every result so you can see what the months amounted to.
Worked examples.
A 90-day deadline
A notice period of 90 days starts on Saturday 7 November 2026. Add 90 calendar days.
Friday, 5 February 2027.
Load this exampleOne month after 31 January
A monthly payment falls due on 31 January 2026. What is the date one month later?
Saturday, 28 February 2026: February has no 31st, so the result moves to its last day.
Load this exampleTen business days over the holidays
An order ships on Friday 18 December 2026 with a 10-business-day delivery promise. The courier is closed on 25 December and 1 January.
Tuesday, 5 January 2027. Both closures fell on weekdays and were skipped.
Load this exampleWorking days in a month
Count the working days from 1 to 30 November 2026, counting both dates, with Thursday 26 November as a public holiday.
20 business days (21 weekdays minus 1 holiday).
Load this exampleHow long until a date?
From 1 January 2025 to 15 March 2026.
438 days, which is 1 year, 2 months and 14 days, or 62 weeks and 4 days.
Load this exampleWhat this calculator
does not do.
- It does not know public holidays for any country. List them yourself in the holiday box.
- It does not handle times of day or time zones. Use the time duration calculator for hours and minutes.
- It uses the modern Gregorian calendar for all years. Historical dates before a country adopted it (1582 in much of Europe, 1752 in Britain and its colonies) may not match period documents.
- Legal and contractual deadlines can have their own counting rules, such as excluding the day of service or moving a deadline off a weekend. Check the rule that applies to you.
Common questions.
Does “days between” include the end date?
Yes, by default the end date is included and the start date is not, so 1 to 4 March is 3 days. Choose “Include start and end” to count both dates (4 days).
What is 31 January plus one month?
28 February, or 29 February in a leap year. When the day number does not exist in the target month, the last day of that month is used.
Are public holidays excluded from business days?
Only the dates you enter in the holiday box. Weekends are always excluded.
Can I subtract from a date?
Yes. Choose “Subtract (before)” in either add mode to count backwards in calendar time or in business days.
Can I share a result?
Yes. The page address updates with your inputs, and “Copy link to these inputs” copies it for you.
- Oracle Java SE 21 — LocalDate.plusMonths (month-end adjustment) — documents the same last-valid-day rule and the ISO proleptic Gregorian calendar
- MDN — Temporal.PlainDate.prototype.add() — the default “constrain” overflow behaviour
- Python documentation — datetime (proleptic Gregorian calendar)
Calculations run in your browser with whole UTC calendar dates, so no time zone can shift a result by a day. Our test suite checks this calculator against independently computed calendar values, including leap years, month ends, early years such as 0099, and randomized comparisons with a day-by-day reference count.