How many days is it from Monday to Friday? Four, if you count the gaps between days. Five, if you count every day you'll be at work. Both answers are right, and mixing them up is the most common way to end up a day off on a deadline, a hotel booking or a project plan.
This guide covers inclusive and exclusive counting, business days, months and leap years, and how legal and contract deadlines are usually counted, with worked examples you can check yourself.
Exclusive vs. inclusive counting
Exclusive counting measures the distance between two dates: how many times you move forward one day to get from the start to the end. Monday to Friday is 4 days. This is what you get when you subtract one date from another in a spreadsheet or in code.
Inclusive counting counts every calendar day from the first to the last, including both. Monday to Friday is 5 days. It's the natural answer for "how many days is the conference?" or "how many days of vacation am I taking?"
The difference is always exactly one day:
inclusive days = exclusive days + 1Our Date Difference Calculator counts exclusively by default and has a "Count the end day too" option for inclusive counting. A few results from it:
| From | To | Exclusive | Inclusive |
|---|---|---|---|
| Mon, Oct 12, 2026 | Fri, Oct 16, 2026 | 4 days | 5 days |
| July 1, 2026 | July 31, 2026 | 30 days | 31 days |
| Jan 1, 2026 | Jan 1, 2027 | 365 days | 366 days |
Which one to use
- Durations and "how long until": exclusive. A countdown from today to an event is the number of days between them.
- Hotel stays: exclusive, counted in nights. Checking in on the 12th and out on the 16th is 4 nights.
- Vacations, events, rentals priced per day, medication courses: usually inclusive, since every day from the first to the last counts.
- Age and anniversaries: exclusive, since you're measuring elapsed time.
- Legal and contractual deadlines: follow the specific rule that applies, covered below.
Counting business days
Business days usually means Monday to Friday, minus public holidays. The calendar part is easy. The holidays are where it gets local.
Take July 2026. Counting every day from July 1 to July 31, there are 23 weekdays and 8 weekend days. But July 4, 2026 falls on a Saturday, and the U.S. Office of Personnel Management's federal holiday schedule lists Independence Day as observed on Friday, July 3. OPM explains that when a holiday falls on a Saturday, "the preceding Friday will be treated as a holiday for pay and leave purposes," and a Sunday holiday moves to the following Monday. So federal offices have 22 working days that month, not 23.
The Date Difference Calculator shows the number of weekdays (Monday to Friday) in any range. It deliberately doesn't remove holidays, because they depend on your country, state, employer and even industry. Subtract the ones that apply to you. Banks, for example, follow the Federal Reserve's holiday schedule, which doesn't always match a company's own days off.
Business days in a spreadsheet
Excel and Google Sheets have NETWORKDAYS(start, end, holidays), which counts weekdays inclusively, meaning both the start and end dates count, and subtracts any dates in a holiday list you provide. WORKDAY(start, n, holidays) goes the other way and returns the date that's n business days after the start. Note that NETWORKDAYS includes both ends, while a plain subtraction like =B1-A1 doesn't, which is a common source of off-by-one errors in the same sheet.
Months are not a fixed number of days
"One month from January 31" has no perfect answer, because February has no 31st. Different tools handle it differently. The Date Difference Calculator treats a month from January 31 as ending on the last day of February, so:
- Jan 31 to Feb 28, 2026: 28 days, shown as "1 month"
- Jan 31 to Mar 1, 2026: 29 days, shown as "1 month, 1 day"
A month can be 28, 29, 30 or 31 days. From October 15 to November 14, 2026 is 30 days but not quite a full calendar month, while November 20 to December 20 is 30 days and exactly one month. When precision matters, such as for interest, notice periods or billing, state the length in days rather than months.
Leap years
In the Gregorian calendar, a year is a leap year if it's divisible by 4, except century years, which are leap years only if they're divisible by 400. The U.S. Naval Observatory describes the rule this way. So 2000 was a leap year, 1900 wasn't, and 2100 won't be. The next leap year is 2028.
The effect on day counts:
- January 1, 2026 to January 1, 2027: 365 days
- January 1, 2028 to January 1, 2029: 366 days
- February 1 to March 1, 2027: 28 days; the same dates in 2028: 29 days
Any calculation that assumes 365 days per year drifts by about a day every four years. Count actual calendar days instead, as date libraries and the calculator do.
How deadlines are counted in U.S. federal courts
Legal deadlines show why the counting rule must be spelled out. Rule 6(a) of the Federal Rules of Civil Procedure says that for periods stated in days, you:
- "exclude the day of the event that triggers the period;"
- "count every day, including intermediate Saturdays, Sundays, and legal holidays;" and
- "include the last day of the period," but if that day is a Saturday, Sunday or legal holiday, "the period continues to run until the end of the next day that is not a Saturday, Sunday, or legal holiday."
Example: a 30-day period triggered on Thursday, October 15, 2026. Day 1 is October 16, so day 30 is Saturday, November 14. Because that's a weekend, the deadline moves to Monday, November 16.
Other courts, agencies, states and contracts use their own rules, and some count only business days. This example illustrates the method. It isn't legal advice, so always check the rule that actually applies to your deadline.
Counting days in code
Most bugs come from two sources: time zones and daylight saving time. If you subtract two timestamps that fall on different sides of a clock change, the difference is 23 or 25 hours, and dividing by 24 gives a fraction. The safe approach is to convert both dates to midnight UTC (or use a date-only type) and then subtract:
const days = (a, b) => Math.round((Date.UTC(b.y, b.m - 1, b.d) - Date.UTC(a.y, a.m - 1, a.d)) / 86400000);
days({ y: 2026, m: 10, d: 12 }, { y: 2026, m: 10, d: 16 }); // 4That's essentially what the calculator does internally. Storing points in time as timestamps has its own limits too. See The Year 2038 Problem, Explained for one of them.
For countdowns to a specific date, our Days Until Calculator shows the remaining calendar days, weeks, weekdays and weekends, updating live.
FAQ
How many days are between two dates if I count both?
Take the plain difference and add one. Monday to Friday of the same week is 4 days apart, or 5 days counting both.
Does "within 30 days" include today?
It depends on who wrote the rule. Many legal and contractual rules exclude the starting day and count from the next one. If it matters, ask or check the exact wording.
How many business days are in a year?
2026 has 261 weekdays. Subtract the public holidays that fall on weekdays where you work: for U.S. federal employees, that's 11 in 2026, leaving 250.
Why does my spreadsheet give a different answer than the calculator?
Check whether each one includes the end date. NETWORKDAYS counts both ends, while plain subtraction gives the distance between the dates, which is one day less. Turn on "Count the end day too" in the calculator to match inclusive formulas.
