📅 Date Calculator
Calculate the number of days between two dates or add/subtract days from a starting date.
Counting days between dates, and adding time to one
Date arithmetic looks trivial until a month boundary, a leap year or a weekend gets in the way. The two operations people need are counting the span between two dates and shifting a date forward or backward by a period, and both have conventions worth knowing before you rely on the answer.
Inclusive or exclusive counting
This is the first thing to settle, because the same two dates give two different answers. From 1 March to 5 March is four days of difference and five days if you count both endpoints. A hotel stay from the 1st to the 5th is four nights. A conference running the 1st to the 5th lasts five days. Contracts and notice periods usually specify which they mean, and when they do not, that ambiguity is where disputes start.
| Method | Result | Typical use |
|---|---|---|
| Exclusive (difference) | 4 days | Hotel nights, interest periods, age |
| Inclusive (both ends) | 5 days | Event duration, holiday entitlement |
| Business days only | 3 or 4 | Notice periods, delivery estimates |
Days in each month
Thirty days has September, April, June and November. All the rest have thirty-one except February, which has twenty-eight, or twenty-nine in a leap year. The knuckle method works too: count across your knuckles and the gaps between them, where knuckles are 31-day months and gaps are shorter ones.
Adding months is not the same as adding days
Adding one month to 31 January gives 28 February, or 29 in a leap year, because 31 February does not exist. Most software clamps to the last valid day. This makes month arithmetic non-reversible: add a month to 31 January and you get 28 February, but subtract a month from 28 February and you get 28 January, not 31. If exact reversibility matters, work in days.
Business days
Counting working days means excluding weekends and public holidays. The weekend part is straightforward: in any seven-day span there are five working days, so a rough count is total days × 5/7. The holiday part is jurisdiction-specific and there is no shortcut. Note that the working week is Monday to Friday in most of the world but Sunday to Thursday in several Middle Eastern countries.
When a contract says "within 30 days", check whether it means calendar or business days. Thirty business days is about six weeks, which is more than twice as long as thirty calendar days once holidays are counted.
Week numbers
ISO 8601 defines week 1 as the week containing the first Thursday of January, with weeks running Monday to Sunday. This means 1 January can fall in week 52 or 53 of the previous year, and a year can have 52 or 53 weeks. The US convention differs, starting weeks on Sunday and defining week 1 as the one containing 1 January, so the two systems routinely disagree by a week at the start of the year.
A worked example
A project starts on 14 January 2026 and has a 90-calendar-day deadline. Ninety days from 14 January is 14 April: 17 remaining days in January, 28 in February, 31 in March and 14 in April. Counted as 90 business days instead, with ten public holidays, the deadline lands around 28 May, six weeks later.
Frequently Asked Questions (FAQs)
Should I count both the start and end dates?
It depends on what you are measuring. Durations like hotel nights and interest periods exclude one endpoint; event lengths and holiday entitlement include both. If a legal document is involved, check whether it says "from" or "on and from", because they differ by a day.
How many business days are in a year?
About 260 before holidays, which is 52 weeks times five. Subtracting public holidays leaves roughly 250 in most countries, though it ranges from about 240 in places with many public holidays to over 255 in those with few.
How do I add months to a date correctly?
Add to the month number and clamp the day to the last valid day of the target month. One month after 31 March is 30 April, not 1 May. Be aware the operation is not reversible, so adding then subtracting a month does not always return the original date.
What is the ISO week date system?
A standard where week 1 is the week containing the first Thursday of January and weeks run Monday to Sunday. It is used widely in business and manufacturing in Europe. Some years have 53 weeks under this definition.
Why do some date calculations differ by one day?
Almost always inclusive versus exclusive counting, or a time zone boundary. If one system stores a date as midnight UTC and another reads it in a local time zone behind UTC, the date can appear a day earlier.
How many days are in a year on average?
365.2425 under the Gregorian calendar, averaged over the 400-year cycle. That is the figure the leap year rules are designed to approximate, and it is why 1900 was not a leap year but 2000 was.