Date Calculator
Move a date forwards or backwards by days, weeks, months, quarters or years to find the date you land on — a deadline 90 days out, a notice period, a renewal. Month lengths and leap years are handled properly, so adding a month to 31 January gives a real date rather than an impossible one. It also measures the gap between two dates if you need that.
Your result will appear here
Pick two dates to see the gap, or add and subtract days, weeks, months, or years from a date.
Which date tool do you need?
This page is for date arithmetic: start from a date, move it, and find where you land. Four neighbouring questions have their own tools.
- How many days between two dates? That is a difference, not date maths — use the Days Between Dates Calculator, which counts days, weeks and business days between two dates and nothing else.
- How old is someone? A birth date to today, expressed in years, months and days: the Age Calculator.
- How long between two times? Hours and minutes rather than days: the Time Duration Calculator.
- Adding hours to a date and time? The Time Calculator works in days, hours, minutes and seconds.
Calendar Days, Weekdays, or Working Days?
“Business days” is the vaguest term in date arithmetic, because it means two different things depending on who wrote the contract. Take 90 of them from Tuesday 15 September 2026 and you get three different answers:
| Counting | What it skips | 90 lands on |
|---|---|---|
| Calendar days | Nothing | Mon 14 Dec 2026 |
| Weekdays | Saturdays and Sundays | Tue 19 Jan 2027 |
| Working days | Weekends and public holidays | Wed 27 Jan 2027 |
Weekends alone move the deadline out by 36 days. Adding the six US federal holidays that fall in that window — Columbus Day, Veterans Day, Thanksgiving, Christmas, New Year’s Day and Martin Luther King Jr. Day — moves it a further 8. So a contract saying “90 business days” without defining the term has a six-week range of possible meanings.
What this tool counts, precisely: the weekend toggle excludes Saturdays and Sundays only. It is a weekday count. No public holidays are excluded — not US federal holidays, not state holidays, not anything. Holiday calendars differ by country, by state, and sometimes by employer, so no general date tool can apply yours. If your deadline turns on working days, count the weekdays here and then subtract the holidays that apply to you by hand.
One more thing this tool does not do: the weekday toggle works in difference mode, counting weekdays between two dates. The add and subtract mode always works in calendar units, so you cannot add 90 weekdays directly. To find a weekday deadline, add calendar days, then check the weekday shown and adjust — or count between candidate dates with the days between dates calculator.
Month Ends, Leap Years and Going Backwards
Adding days is easy. Adding months is where date arithmetic stops being obvious, because months are not a fixed length and the answer has to stay a real date.
The convention used here, and by most software, is to keep the same day of the month and clamp to the last day when that day does not exist:
| Start | Change | Result |
|---|---|---|
| 31 Jan 2026 | +1 month | 28 Feb 2026 — there is no 31 February |
| 31 Mar 2026 | −1 month | 28 Feb 2026 — clamping works both ways |
| 29 Feb 2024 | +1 year | 28 Feb 2025 — 2025 has no 29 February |
| 31 Aug 2026 | +6 months | 28 Feb 2027 |
This makes month arithmetic asymmetric. 31 January plus one month is 28 February, but 28 February minus one month is 28 January, not 31 January. You do not always get back where you started, which matters when a renewal date is calculated forward and then checked backward.
Leap years follow the Gregorian rule: divisible by 4, except century years, unless divisible by 400. So 2000 was a leap year, 2100 will not be, and 2026 is not. Any span crossing a 29 February is one day longer than the same span elsewhere, which is why counting in days is safer than counting in months whenever the exact number matters.
Negative intervals are just subtraction: enter a date before your start and the difference comes back the same size, measured the other way. Subtracting works symmetrically for days and weeks — it is only months and years where the clamping above applies.
Quick Answer
To find a date in the future, set your start date, choose a unit and enter how many to add — 90 days from 15 September 2026 is 14 December 2026, a Monday. Subtract instead to go backwards. Everything here is in calendar days, which count weekends and public holidays alike. Counting only weekdays instead would put the same 90 at 19 January 2027, and dropping US federal holidays as well would push it to 27 January.
How It Works: Formula & Variables
Difference = End Date − Start Date · New Date = Start Date ± Calendar Units
- Common year
- 365 days.
- Leap year
- 366 days — divisible by 4, except century years that are not divisible by 400.
- ISO week
- Monday to Sunday, numbered 1–53; week 1 is the week containing January 4.
- Formats
- US (mm/dd/yyyy), EU (dd/mm/yyyy), and ISO 8601 (yyyy-mm-dd) are all shown.
Worked Examples
Example 1: A deadline 90 calendar days out
A contract signed on Tuesday 15 September 2026 gives you 90 days to respond. Set the start date, choose days, add 90: the deadline is Monday 14 December 2026. Where a deadline lands on a weekend it often rolls to the next working day — the calendar cannot tell you that, only the contract can.
Example 2: The same 90, counted as weekdays
If that contract said 90 business days and means weekdays, the answer moves to Tuesday 19 January 2027 — 36 days further out, because you skip roughly two days in every seven. If it also excludes US federal holidays, it moves again to Wednesday 27 January 2027. Same number, same start date, three deadlines six weeks apart. This tool counts the middle one: weekdays, with no holidays excluded.
Example 3: A month-end renewal
A subscription starting 31 August 2026 renews every six months. Add six months and the renewal is 28 February 2027, not the 31st, because February has no 31st and the date clamps to the month end. Add another six and you get 28 August 2027 — the original 31st does not come back. Anything billing on the 29th, 30th or 31st drifts this way.
Example 4: Measuring a span that crosses a leap day
From 4 September 1999 to 2 January 2003 is 1,216 days, or 1,217 counting the end date as well — about three years and four months. The span includes 29 February 2000, so it is one day longer than the same three-and-a-bit years starting a year later. If you only need this kind of figure, the days between dates calculator is the more direct tool.
Key Concepts
Months are not a fixed length: They run from 28 to 31 days, so a "one month" jump lands on a different day depending on where you start. Adding a month to January 31 lands at the end of February, not on a 31st that does not exist.
Including the end date shifts the total by one: Whether you count the last day changes the answer by exactly one day, so it helps to be clear about which convention a deadline uses.
Date formats are easy to misread: 03/04/2026 is March 4 in the US notation and April 3 in the EU one. The result panel spells the date out and shows all three formats to remove the doubt.
Common Mistakes
Treating every month as 30 days: Month lengths vary, so a rough "30 days a month" estimate drifts further off the longer the span.
Forgetting leap years: Skipping February 29 quietly undercounts any range that crosses a leap day.
Mixing up US and EU notation: Reading 03/04/2026 in the wrong order shifts a date by a month or more.
Frequently Asked Questions
Enter both dates and the calculator counts the actual calendar days between them, so leap years and the different lengths of each month are already handled for you. You also get the gap expressed in years, months, and weeks.
Switch to the "Add / subtract" tab, put in a starting date, choose days as the unit, and enter the number you want to move forward. Pick "Subtract" instead to count backwards to an earlier date.
By default the end date is included in the total, which is what most people expect when they ask how many days something covers. Untick that option to count the gap between the dates instead, which comes out one day shorter.
A year is a leap year when it divides evenly by 4, except for century years, which only count if they also divide by 400. That is why 2000 was a leap year but 1900 was not.
ISO weeks run Monday to Sunday and are numbered 1 to 53, and week 1 is always the week that contains January 4. The "Add / subtract" result shows the ISO week for the date you land on.
The plain day count includes every calendar day. Tick "business days only" to also see the count with Saturdays and Sundays removed. Public holidays vary by country, so they are not removed automatically.
Related Calculators
Time Duration Calculator
Measure the difference in hours and minutes between two times, not the gap in days.
Time Calculator
Add or subtract time in days, hours, minutes, and seconds, including applying time to a date.
Days Between Dates Calculator
A focused count of days, weeks, or business days between two dates.
Age Calculator
Work out an exact age in years, months, and days from a birth date.
Standards and assumptions
- Calendar. Proleptic Gregorian throughout. Leap years are those divisible by 4, excluding century years not divisible by 400 — 2000 was a leap year, 2100 will not be.
- Week numbering and date format. ISO 8601: weeks run Monday to Sunday, and week 1 is the week containing 4 January. Dates are shown in US (mm/dd/yyyy), EU (dd/mm/yyyy) and ISO (yyyy-mm-dd) form together, because 03/04/2026 is genuinely ambiguous on its own.
- Month arithmetic. The day of the month is preserved and clamped to the last day where it does not exist, which is the convention used by most date libraries. It is a convention, not a law — some financial contracts define month-ends differently, and the contract wins.
- Weekdays. The weekend toggle excludes Saturdays and Sundays only, so it produces a weekday count rather than a working-day count. No public holidays are excluded — they vary by country, state and employer, and no general tool can know yours. Where this page says “business days” it means weekdays; adjust for holidays by hand.
- Time zones and daylight saving. Not applied. All arithmetic is on whole calendar dates, so a date is a date wherever you are.