Counting the days between two dates
Both counts are shown because both are correct — which one you want depends on the question. From 1 March to 10 March the exclusive gap is 9 days: that is how many midnights lie between the two dates, and it is what “how many days until” means. The inclusive count is 10, because it counts the first day and the last day as well, which is how a hotel stay, a rental period or a contract term is normally read.
The order of the two dates does not matter. If the end date is earlier than the start date the tool takes the absolute difference and marks the range as reversed instead of handing you a negative number to interpret. Weeks come from the same total: 100 days is 14 weeks and 2 days, because 100 ÷ 7 is 14 remainder 2.
All of it is whole-day arithmetic performed at UTC midnight, so a result never shifts because of a time zone or a daylight-saving change — a problem that shows up in calculators built on local timestamps, where a range can come out one day short twice a year. The weekday shown beside each date is the weekday of that calendar date anywhere in the world.
Years, months and days — and why the breakdown surprises people
Next to the day count the same span is shown as years, months and days. It is found by walking forward from the start date: add the whole years that fit, then the whole months, then count the days left over. From 31 January 2024 to 1 March 2024 that is 1 month and 1 day — the walk lands on 29 February (the last day February has) and then counts one more day.
These are real calendar months, never total days divided by 30. Because months differ in length, the same “one month” covers 31 days from 1 January and 28 days from 1 February. The two figures answer different questions and neither can be converted into the other without knowing where in the calendar the span sits.
That is also why a rule saying “within 30 days” and one saying “within one month” are not the same rule. 45 days from 1 January is 1 month and 14 days, while 45 days from 1 February is 1 month and 17 days. When a deadline matters, count in the unit the document uses.
Business days: exactly what gets skipped
The business-day count walks the range one day at a time and drops every Saturday and Sunday. With a holiday preset selected it drops that country’s public holidays too, and because each date is visited once, a holiday that falls on a weekend can never be subtracted twice. Every holiday inside your range is listed under the result with its date, so the number can be checked rather than trusted.
- United States: the 11 federal holidays, each on its observed date — New Year’s Day, Martin Luther King Jr. Day (3rd Monday of January), Presidents’ Day (3rd Monday of February), Memorial Day (last Monday of May), Juneteenth, Independence Day, Labor Day (1st Monday of September), Columbus Day (2nd Monday of October), Veterans Day, Thanksgiving (4th Thursday of November) and Christmas. A fixed-date holiday on a Saturday is observed the Friday before, one on a Sunday the Monday after.
- United Kingdom (England and Wales): the 8 bank holidays — New Year’s Day, Good Friday, Easter Monday, the early May bank holiday (1st Monday of May), the spring bank holiday (last Monday of May), the summer bank holiday (last Monday of August), Christmas Day and Boxing Day. Easter is computed with the anonymous Gregorian computus, and a fixed-date holiday landing on a weekend moves to the next free weekday — which is why Christmas on a Sunday is substituted on the 27th while Boxing Day keeps the 26th.
Korean, Japanese and other national calendars are not built in. Seollal and Chuseok move with the lunar calendar and substitute-holiday rules change by legislation, so a generated list would be wrong in some years — and a wrong holiday is worse than no holiday. Use “Weekends only” and subtract the public holidays yourself. The business-day count is also skipped for ranges longer than 36,525 days (about 100 years), because the walk is day by day.
Adding and subtracting: the order is fixed, and it matters
Offsets are applied in one order — years, then months, then a clamp to the last day of the target month, then weeks, then days. So 31 January plus one month is 28 February (29 February in a leap year), and 31 January plus one month and one day is 1 March, because the clamp happens before the day is added.
One consequence is worth remembering: the operation is not associative. Adding one month twice to 31 January gives 28 March (31 Jan → 28 Feb → 28 Mar), while adding two months at once gives 31 March. Neither is a bug — “one month later” simply has no single definition when the month is short. If the contract counts in days, use the Days field and the question disappears.
Turn a business-day setting on and “Add or subtract” switches to a working-day step: the Years, Months and Weeks fields are ignored and only Days is used, stepping one calendar day at a time and counting only the days that are neither a weekend nor a holiday. That is what “10 working days from today” means. Calendar offsets accept up to 100,000 in each field; the business-day offset stops at 20,000 working days.
Deadlines people count wrong
Day 0 or day 1. “Within 7 days” of something that happened on a Monday normally ends the following Monday, not Sunday, because the day of the event itself is not counted. Korean civil law says so explicitly — 민법 제157조, 초일불산입: the first day of a period is not counted unless the period starts at midnight. That is the exclusive count on the first card; the inclusive count is the one behind a hotel stay or a festival that “runs for 3 days”.
D-Day counting is different again. In Korean usage the day of the event is D-Day and the day before is D−1, while an anniversary counted in days treats the first day as day 1 — so the “100th day” is the start date plus 99 days, not plus 100. Use the inclusive count for that, or add 99 days on the second tab.
Working days are not calendar days. Ten working days added to a Monday lands on the Monday two weeks later — 14 calendar days, not 10 — and a single public holiday inside that stretch pushes it to the Tuesday. Quote turnaround times in the unit you actually mean, and check the holiday list the tool prints underneath before you promise a date.
Numbers worth remembering
- 30 days is not one month: 30 days from 31 January is 2 March in a common year, while one month from 31 January is 28 February.
- 90 days from 1 January is 1 April (31 + 28 + 31 = 90 in a common year) — the classic “90-day notice” trap when people write “three months”.
- A common year is 365 days = 52 weeks + 1 day; a leap year is 366 = 52 weeks + 2 days. That is why a fixed date moves one weekday later each year, and two across a leap day.
- 1,000 days from 1 January 2026 is 27 September 2028; 10,000 days is about 27 years and 4 months.
- A year holds 260 to 262 weekdays before any public holiday is taken out; the US federal calendar removes 11 of them and the England and Wales calendar 8.
Limits and edge cases
- Dates before 1583 are refused. The Gregorian calendar began in October 1582 and countries adopted it decades apart, so the same written date can mean two different days depending on the record.
- Fractional offsets are rejected. “1.5 months” has no calendar meaning; write it as weeks and days instead.
- A blank start date means today, taken from your device’s clock. Fill it in whenever you need a result that stays the same tomorrow — and before you share the link.
Everything is computed in this browser and nothing is uploaded. Share link puts the dates and options into the URL so the page reopens exactly as you left it, which is also why a link with a private date in it is worth keeping off public pages.