Date (Days Between) Calculator
Count the days between two dates, or add and subtract days, weeks and months from one.
What do you need?
This finds the number of days between two dates, or the date a given number of days, weeks or months from another.
It handles leap years and can count business days only.
How date arithmetic works
Counting days between two dates is simple in principle and awkward in practice, because months are not a fixed length and leap years follow a rule with an exception inside an exception.
The leap rule is: divisible by 4, except century years, unless those are divisible by 400. So 1996 and 2000 were leap years, 1900 was not, and 2100 will not be. Most people know the first part and not the rest.
The subtler problem is that "three months from now" is genuinely ambiguous. Three months from 31 January could be 30 April or 1 May, depending on whether you clamp to the end of the month or overflow into the next. Different software makes different choices, which is why two tools can disagree.
1900 failed at the third test and was a common year. 2000 passed it and was a leap year. 2100 will fail it.
What to enter
- Start and end date
- For counting the days between them. Whether both endpoints are counted is a choice the tool makes explicit.
- Number of days, weeks or months to add
- For finding a future or past date. Days and weeks are unambiguous; months are not.
- Business days only
- Counts weekdays and excludes weekends. Public holidays vary by country and are not assumed.
Things that make dates awkward
- Month lengths
- 28 to 31 days. There is no fixed number of days in a month.
- Leap years
- Divisible by 4, except centuries, unless divisible by 400.
- Inclusive vs exclusive counting
- Whether both endpoints count. A five-day event runs Monday to Friday, which is four days apart.
- Business days
- Weekdays only. Public holidays differ by country and often by state.
- Time zones
- The same instant can be two different dates in two places.
What this assumes
Business day counts exclude weekends but not public holidays, since those vary by jurisdiction.
Adding months clamps to the end of the target month, so 31 January plus one month gives 28 or 29 February rather than overflowing into March.
How to calculate date calculations
Convert both dates to a day count, subtract, then convert back if needed.
- leap years
- Add a day for each 29 February that falls in the range
- inclusive
- Add one if both endpoints should count
Decide whether both endpoints count. Monday to Friday is four days apart and five days inclusive. Contracts and deadlines usually specify which.
Count whole days, allowing for leap years. Any range spanning a 29 February gains a day.
For business days, exclude weekends. And subtract public holidays manually, since they differ by country and often by state.
Be careful adding months. Days and weeks are unambiguous. Months are not, so check what a tool does at month ends before relying on it.
See a worked example: counting to Christmas
- From
- 1 January 2026
- To
- 25 December 2026
2026 is not a leap year, since it is not divisible by 4.
Days between: 358.
Inclusive of both endpoints that would be 359.
As business days it drops to roughly 256, before subtracting whatever public holidays apply where you are.
358 days
Frequently asked questions
Divisible by 4, except century years, unless those are divisible by 400.
So 2024 was a leap year, 1900 was not, and 2000 was. The next century year to be skipped is 2100. The rule exists because a solar year is about 365.2422 days, not 365.25.
Usually inclusive versus exclusive counting. Monday to Friday is four days apart, or five days inclusive of both.
The other cause is month arithmetic. Adding one month to 31 January gives 28 February in some systems and 3 March in others, and neither is wrong exactly.
Count weekdays and exclude Saturdays and Sundays. A full year has about 260 weekdays before holidays.
Public holidays have to come off separately, because they vary by country and often by state. Most tools cannot assume them for you.
There is no fixed answer, which is the whole difficulty. Months run 28 to 31 days.
For approximations, 30.44 days is the average month length. For anything exact, count actual calendar days rather than multiplying.
Subtract the birth year, then subtract one more if the birthday has not yet occurred this year. That gives whole years.
For years, months and days, work down in that order: full years first, then remaining months, then remaining days. Doing it in a different order gives inconsistent results at month ends.
The international date format: YYYY-MM-DD, so 2026-12-25.
It is unambiguous and sorts correctly as text, which is why it is standard in software. It also avoids the US MM/DD versus European DD/MM confusion, where 03/04 could be two different dates.
Problems people actually run into
Assuming every fourth year is a leap year
The century exception catches people out. 1900 was not a leap year and 2100 will not be, despite both being divisible by 4.
Software that gets this wrong is rarer than it used to be, but the rule is worth knowing when checking a date range by hand.
Mixing inclusive and exclusive counting
"Five business days" and "by the fifth business day" can mean different deadlines, and contracts frequently do not specify.
State which you mean when it matters. A one-day difference on a filing deadline is not a rounding error.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026