Date Calculator

What this calculator does

Date questions come in three shapes, and each shape has its own traps. How long between two dates: the trap is uneven months and leap years. What date is it 90 days, or 6 months, from now: the trap is month-end, where adding a month to January 31 cannot land on February 31 because there is no such day. How many working days are in this period: the trap is that weekends and holidays are not the same in every country, company or faith, so any tool that hard-codes one country's holidays is quietly wrong for everyone else. This calculator gives each shape its own mode, handles the traps openly, and states its conventions where you can see them instead of burying them in fine print.

How to use it

1. Days between two dates counts the span the standard way: the number of midnights crossed, so 13 October to 14 October is 1 day. If you need the count that includes both the first and the last day, as hotel nights and many booking systems count differently again, tick the include box and the page shows that figure as its own clearly labelled line rather than changing the main one behind your back. Dates entered backwards are swapped with a note, not an error, because the span is the same either way.
2. Add or subtract takes years and months first, then weeks and days, which is the order that matches how people mean it. The month-end rule is a clamp: January 31 plus one month lands on the last day of February, 28th or 29th depending on the year. Repeat applies the same step again and again for schedules (every two weeks, monthly billing), lists each step, and reports the total calendar days actually moved, which for month steps is not simply the step size times the count, because months differ in length.
3. Business days counts the days after the start date up to and including the end date, removes your chosen weekend pattern, then removes your holidays, but only the ones that were not already weekend days: a holiday on a Saturday removes nothing, and the results show that distinction as its own line. Paste your holidays one per line in the format shown. There is deliberately no built-in holiday list, because a list for the wrong country is worse than no list.
4. Weekdays are always shown for the start and end dates, because half of all date mistakes are really weekday mistakes: the deadline that fell on a Sunday, the anniversary checked against the wrong year.

How it is calculated

Every date is converted to a calendar serial number (days since a fixed point) using the real Gregorian calendar. Differences are subtractions of serial numbers, which is why they are exact across leap years. Years and months are added on the calendar itself, clamping the day to the last day of the target month when the original day does not exist there. Weeks and days are added as serial numbers. Weekday and business counts walk the actual days in the span and classify each one. No average month or year length is used anywhere.

Worked examples

A quarter's span: 13 October 2026 to 13 January 2027 is 92 days, exactly 0 years, 3 months, 0 days in calendar terms, or 13 weeks and 1 day. Inside it sit 66 Monday-to-Friday weekdays and 26 weekend days. Ticking include-the-end-date gives 93, which is the count of calendar dates touched rather than midnights crossed: both numbers are right, for different questions, and the page labels which is which.

The month-end clamp: 31 January 2026 plus one month is Saturday 28 February 2026, not an error and not March 3. Subtracting one month from 31 March 2026 lands on the same 28 February. This clamp is the convention spreadsheets use, and stating it matters because other conventions exist (some systems roll over to March 1) and the two disagree by days.

A leap-day start: 29 February 2024 plus one year and one month, applied one step at a time, is Friday 28 March 2025: the year step clamps to 28 February 2025 first, and the month step then runs from there.

A payroll fortnight, repeated: starting Tuesday 13 October 2026 and adding two weeks three times gives 27 October, 10 November and Tuesday 24 November 2026, 42 calendar days moved. Week-based steps are the well-behaved ones: they never clamp, never drift, and always land on the same weekday.

Business days: Tuesday 13 October to Tuesday 20 October 2026 is 7 calendar days. With a Saturday-Sunday weekend, 2 are weekend days and 5 are business days. Add Monday 19 October to the holiday list and business days fall to 4, with the results showing 1 holiday inside the span and 1 that removed a business day, because the 19th was a working Monday.

Reading your result

If a span looks one day short, you are almost certainly looking at the inclusive and exclusive counts disagreeing, which is the oldest argument in date arithmetic. Deadlines that say "within 30 days of" usually mean the midnight count. Bookings that charge per night count differently again. Decide which question you are asking, then read the line that answers it; the page gives you both rather than guessing. If a business-day count surprises you, work down its lines: calendar days, minus weekend days, minus effective holidays. Each line can be checked by hand on a calendar in under a minute, which is the standard every figure here is built to meet.

Limitations

• Not a legal deadline engine. Courts, tax authorities and regulators count days with jurisdiction-specific rules: what happens when a deadline lands on a holiday, which holidays count, whether the trigger day is included. This calculator does plain calendar arithmetic and says so. For a filing date that carries a penalty, use the official rules of the body you are filing with.
• Your holidays, your responsibility. Business day mode is only as right as the list you paste in. It deliberately ships no national list, so it cannot silently apply the wrong country's calendar, but it also cannot warn you about a holiday you forgot.
• The clamp convention is a choice. Month arithmetic here clamps to the end of a short month, matching spreadsheets. The Age Calculator on this site uses a different rule for February 29 birthdays specifically, because birthdays are observed, not computed; each page states its own convention where it applies.
• Dates only, no times. Spans are whole calendar days. For times of day, durations and clock arithmetic, the Time Calculator is the right tool.

Frequently asked questions

How many days are there between two dates, including both ends?

Take the ordinary difference and add one. That is what the include box does. It is the right count when both the first and last days are part of the thing being counted, like the days of a festival or a stay counted in calendar days. It is the wrong count for most deadlines and durations, which count the midnights between the dates. The page shows the main count always and the inclusive count when you ask for it, labelled, so the two are never confused.

Why does adding a month sometimes add 28 days and sometimes 31?

Because months are not a unit of fixed length; they are named stretches of the calendar. Adding a month means the same date next month, and when that date does not exist, the clamp rule lands on the month's last day. The total days moved line in the results exists precisely to expose this: a six-month step can move 181, 182, 183 or 184 days depending on which months it crosses and whether a February is involved.

How do I count working days for a leave request?

Use Business days mode with your weekend pattern and your employer's holiday list for your location, pasted in the box. Count the days after your first day of leave through your last day, which is the span this mode measures. Then sanity-check the answer against its own breakdown lines. If your employer counts the first day differently, adjust the start date by a day and note that their convention, not the arithmetic, is what differs.

Can it handle dates far in the past or future?

Differences and additions work for any dates the picker accepts, using the Gregorian calendar projected in both directions, which is the standard convention. The day-by-day counts (weekdays and business days) are capped at spans of 100 years, and the page tells you if you ask for more.

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Method: calendar serial day numbers for all differences and day steps; years and months applied on the calendar with a month-end clamp; weekday and business counts by walking the actual days in the span; inclusive counting offered as a separate labelled figure. Last reviewed: 13 October 2026.