Date difference calculator: days between two dates

The gap between two dates has more than one correct answer, and the disagreements are not rounding noise. A stay from March 2 to March 5 is three nights or four days depending on what you are counting, and a contract that says "within 30 days" can mean two different deadlines depending on whether the first day counts.

This calculator takes a start date, an end date and a switch that decides whether both the first and last day are included. It returns the total days, the years-months-days breakdown, weeks, whole weeks plus leftover days, approximate months and years, weekdays only, weekend days, total hours, and the day of the week at each end.

What follows is the arithmetic behind each of those outputs, where the conventions collide, and how to pick the one your situation actually requires.

#The formula

The core figure is a difference of day numbers. Convert each date to a serial day count -- the number of days since a fixed reference date -- and subtract.

Formula: total days = serial(end) - serial(start), plus 1 if both endpoints are counted. The breakdown uses component subtraction with borrowing: years = Y2 - Y1, months = M2 - M1, days = D2 - D1; if days < 0, add the length of the month before the end date and reduce months by 1; if months < 0, add 12 and reduce years by 1.

Symbols: Y1-M1-D1 is the start date, Y2-M2-D2 the end date. The serial conversion is what handles leap days without any special case -- February 29 simply occupies a serial number like every other day.

The derived rows are all functions of the day total: weeks = days / 7, whole weeks and a remainder from days // 7 and days % 7, hours = days x 24, approximate months from days / 30.4375 and approximate years from days / 365.25. The divisor 30.4375 is 365.25 / 12, the mean month length under the leap-year cycle.

#A worked example, done by hand

Worked example: March 2, 2026 (a Monday) to May 15, 2026 (a Friday).

1. Days remaining in March after the 2nd: 31 - 2 = 29. April contributes 30. May contributes 15. Total: 29 + 30 + 15 = 74 days, exclusive of the start date.
2. With both endpoints counted, add one: 75 days.
3. Breakdown: 5 - 3 = 2 months and 15 - 2 = 13 days, with no borrowing needed, so 2 months and 13 days.
4. Weeks: 74 / 7 = 10.57, or 10 whole weeks and 74 - 70 = 4 days.
5. Weekdays: 10 whole weeks contribute 10 x 5 = 50 weekdays and 20 weekend days. The 4 leftover days run Monday May 11 through Thursday May 14, all weekdays, so 50 + 4 = 54 weekdays and 20 weekend days.
6. Hours: 74 x 24 = 1,776.

Every one of those steps is reproducible on paper. The only piece that needs a calendar is step 5, because the leftover days must be walked through individually.

#Inclusive against exclusive counting

Exclusive counting -- the default -- measures the interval between the two dates, which is the number of nights. Inclusive counting measures the number of calendar days touched, which is what you want when each day is a unit of service or occupancy.

The off-by-one is not a rounding question, it is a definition question, and it is expensive in exactly the places where nobody checks. A notice period of "30 days from March 2" ends April 1 under exclusive counting and March 31 under inclusive counting. A billing period stated inclusively on one side and exclusively on the other double-counts a day every cycle.

Note: when a document says "from" a date, exclusive counting is the more common legal reading, and "beginning on" usually signals inclusive counting. The document controls, not the arithmetic.

#Why "months between two dates" has no single answer

A month is not a fixed quantity. Three reasonable methods give three different answers for the same pair of dates, and none of them is wrong.

The first two rows are the interesting ones. Most people would call January 31 to February 28 "one month" because it runs end of month to end of month, but the component arithmetic says zero months and 28 days, because February never reaches a 31st. The same happens from August 31 to September 30.

So there are at least three conventions in play: end-of-month matching, same-day-of-month anniversary, and an average-length divisor. Subscription billing usually uses end-of-month matching, loan schedules usually use the anniversary rule, and statistical work usually uses the divisor. Check which one applies before quoting a figure in months.

#Counting weekdays, and why holidays are not automatic

The weekday count uses the structure of the week rather than a day-by-day loop for the bulk of the work.

1. Divide the total days by 7. Each whole week contributes exactly 5 weekdays and 2 weekend days, regardless of where it starts.
2. Walk the remaining days % 7 days individually, starting from the day of the week of the start date, and count how many fall Monday through Friday.
3. Weekend days are the total minus the weekdays.

That is why the answer here counts Saturdays and Sundays only. Public holidays cannot be included automatically because they are not a property of the calendar -- they are a property of a jurisdiction, an employer and sometimes a contract. Federal holidays differ from state holidays, several move with the day of the week, some are observed on an adjacent weekday when they fall at the weekend, and private employers are not obliged to follow any of them. Subtract your own holiday list from the weekday figure.

#Day-count conventions in finance

Finance solved the same ambiguity by writing the convention into the contract. The two parts are the numerator, meaning how days are counted, and the denominator, meaning how many days a year is deemed to have.

Assume a balance of $10,000 and an assumed nominal rate of 6% a year. The interest for the same calendar span differs by convention:

The 30/360 convention pretends every month has 30 days and every year 360, which makes coupon payments equal and schedules tidy. Actual/360 counts real days but divides by 360, which quietly raises the effective rate by a factor of 365/360, about 1.4%. Actual/365 counts real days over a real year.

None of this is a trick; it is disclosed in the terms. It is worth knowing because it is the same ambiguity this page is about, priced in dollars. The loan calculator and the credit card payoff calculator both assume regular monthly periods, which is the simplification most consumer schedules use.

#Where each input comes from

Both dates should come from the document that governs the question -- the contract date, the invoice date, the start date on an offer letter -- not from memory. If a document gives a date without a year, resolve the year before entering anything.

The inclusive switch is the input that changes the answer most, and it is the one people skip. Set it by asking whether the first day is itself a unit of the thing you are counting. If the answer is nights, interest days or elapsed time, leave it off. If the answer is days of service, days of attendance or days of leave, turn it on.

Order does not matter: reversing the dates gives the same magnitude, with the calculator noting that the dates were swapped.

#How to read the result

The total days figure is the exact one. Everything below it is either a repackaging of that figure or an approximation, and the approximations are labeled.

Take the weeks row as a decimal for planning capacity and the whole-weeks-plus-days row for scheduling, since work rarely happens in fractions of a week. The approximate months and years rows use average month and year lengths and will not agree with the borrowing breakdown; the breakdown is the one to quote when precision matters.

The weekday count is the figure that maps onto a working schedule. Multiply it by the daily hours from the time card calculator to convert a date range into a labor budget, or feed it into the PTO accrual calculator when the range covers a leave period.

#What this model leaves out

• Time of day. The calculation compares dates, not timestamps. A span from 11 pm on one day to 1 am the next counts as one day, not two hours.
• Time zones. Two events an hour apart can fall on different calendar dates in different zones. Fix a single zone before counting.
• Daylight saving. A civil day is not always 24 hours. In regions that observe it, one day a year has 23 hours and one has 25, so the hours row can be off by an hour either way across a spring or fall transition.
• Holidays and closures. Weekday counts exclude weekends only.
• Business-day rolling. Financial contracts often shift a date that lands on a weekend to the next or previous business day. That adjustment is not applied here.
• Historical calendars. Dates before the local Gregorian adoption do not line up with modern serial day numbering.

#Common mistakes

Counting inclusively on one side only. If the start date counts as day one, the end date must be included too. Mixing the conventions inside one calculation is the source of most billing disputes over a single day.

Treating "30 days" as "a month". They are equal in exactly four months of the year. For a deadline stated in days, count days.

Assuming 52 weeks is a year. 52 x 7 = 364, so a 52-week schedule is a day short of a common year and two days short of a leap year. Over five years that drifts by nearly a week.

Reading weekday counts as business days. They are not the same wherever holidays exist.

Forgetting February 29. Any span that crosses a leap day is a day longer than the same span in a common year, which matters for daily-rate billing. The age calculator shows the same effect over decades.

Rounding months to compare durations. Two spans both reported as "2.4 months" can be several days apart. Compare the day totals.