Date Calculator
Add or subtract years, months, weeks or days from a date, or count the time between two dates. Handles leap years, month-end clamping, business days and daylight saving correctly.
Add to or subtract from a date
Time between two dates
For business days and US federal holidays, use the days between dates calculator.
- Weeks
- 14 w 3 d
- Weekdays
- 73
- Weekend days
- 28
- Months
- 3
How date arithmetic works
Two different questions
"How many days" and "how many months and days" are different questions with different answers, and both are correct. A day count is exact and unambiguous. A calendar breakdown depends on which months the span crosses, because months vary from 28 to 31 days. This calculator shows both so you can use whichever your situation calls for.
Month-end clamping
Adding a month to 31 January has no exact answer, because February has no 31st. The universal convention is to clamp to the last valid day of the target month:
31 Jan 2026 + 1 month = 28 Feb 2026 31 Jan 2028 + 1 month = 29 Feb 2028 (leap year) 31 Mar 2026 + 1 month = 30 Apr 2026
A consequence worth knowing: the operation is not reversible. Add a month to 31 January and subtract one again and you land on 28 January, not the 31st. That is expected behaviour, not a bug, and it is why contracts usually specify "the last day of the month" explicitly.
Why the calendar breakdown is computed, not subtracted
The obvious way to break a span into years, months and days is to subtract each field and borrow when one goes negative, like long subtraction. That fails when the start day is longer than the month it borrows from: from 31 January to 1 March, borrowing February's 28 days against a deficit of 30 still leaves a negative number. This calculator instead finds the largest whole number of months that still lands on or before the end date, then counts the remaining days exactly — which guarantees that adding the breakdown back to the start date returns the end date.
Leap years
leap = (year divisible by 4) AND (not divisible by 100 OR divisible by 400)
The century rule is the one that catches people out: 1900 was not a leap year, 2000 was, and 2100 will not be. The Gregorian calendar adds this correction because the tropical year is about 365.2422 days, so a plain four-year rule drifts by roughly three days every four centuries.
Frequently asked questions
How many days are between two dates?
Subtract the earlier date from the later one and count whole days. From Tuesday, September 15, 2026 to Friday, December 25, 2026 is 101 days — 14 weeks and 3 days, or 3 months and 10 days on the calendar. The two readings differ because months are not a fixed length.
Should I count the end date or not?
It depends what you are measuring. "Days until" is exclusive: Monday to Friday is 4 days. "How long is my trip" is inclusive and counts both endpoints: Monday to Friday is 5 days. Payroll, hotel stays and notice periods usually count inclusively; deadlines and countdowns usually do not. Use the "count both start and end day" switch to pick.
What happens when I add a month to the 31st?
The result is clamped to the last day of the target month, so 31 January plus one month is 28 February (29 in a leap year), not 3 March. This is the convention used by spreadsheets, databases and every major date library. It also means the operation is not reversible: adding a month to 31 January and then subtracting one returns 28 January.
In what order are years, months and days applied?
Calendar units first, then exact days. Adding "1 month and 1 day" to 31 January gives 28 February after the month shift, then 1 March after the day. Applying them the other way round would give a different answer, which is why the order is fixed and stated rather than left to chance.
What counts as a business day?
Monday to Friday. Tick "Also skip US federal holidays" to skip those too — on the day offices actually close, so a holiday that falls on a Saturday is skipped on the Friday before and a Sunday holiday on the Monday after. The holidays skipped are listed under the result. A week counts as five business days; years and months are calendar units, so they are turned off in this mode. The start date itself is not counted, the same as Excel's WORKDAY function. State, local and company holidays vary too much to assume, so they are not included.
Does daylight saving time affect the result?
No. All arithmetic here is done on calendar dates rather than on timestamps. A common bug in date tools is adding 86,400,000 milliseconds per day, which lands on the wrong calendar day when a clock change makes a day 23 or 25 hours long. This calculator never touches a clock, so spring-forward and fall-back weekends give the same answer as any other week.
How are leap years handled?
A year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2000 was a leap year and 1900 was not; the next is 2028. Leap days are accounted for in every day count, and adding a year to 29 February clamps to 28 February in the following non-leap year.
Is my data sent anywhere?
No. Every calculation runs in your browser. Nothing you enter is uploaded, stored or logged, and the page works offline once loaded.