Date Difference Calculator
Calculate the time between two dates in years, months, weeks, days and working days, with the option to include or exclude the end date.
How to use this calculator
- 1Enter the two dates in any order — the calculator handles reversed dates.
- 2Switch on "count the end date" when you need an inclusive count, such as the number of days in a hotel stay or a project schedule.
How the calculation works
Total days = (End − Start) / 86,400,000 ms Calendar span = years, then months, then days- Total days
- Simple elapsed days between the two dates
- Calendar span
- The same period expressed in calendar units
The two answers are both correct and mean different things. "94 days" is an elapsed count; "3 months and 3 days" is the calendar span, and the number of days in those months varies.
Working days count Monday to Friday and do not exclude public holidays, which differ by country.
All arithmetic is done in UTC, so daylight-saving changes cannot introduce an off-by-one-day error.
Worked example
From 10 August 2026 to 25 December 2026
- 1.From 10 August to 10 December is 4 complete months.
- 2.From 10 December to 25 December is a further 15 days.
- 3.The calendar span is therefore 4 months and 15 days.
- 4.In elapsed terms that is 137 days, of which 98 are weekdays.
Result: 4 months, 15 days — 137 days total
Two honest answers to "how long is that"
Ask how long the gap is between two dates and there are two different, equally correct answers. One is an elapsed count — the number of individual days that separate them, useful for anything measured in raw time. The other is a calendar span — years, months and days, the units people actually use when they talk about durations out loud. A hundred and thirty-seven days and "four months and fifteen days" can describe the exact same pair of dates, because calendar months are not a fixed number of days.
Neither answer is more correct than the other; they simply answer different questions. A shipping estimate or an interest calculation wants the elapsed count, in days. A description of how long a project ran, or how old an agreement is, is usually easier to picture as a calendar span — which is why this calculator reports both at once.
Where date-difference calculations show up
Counting the gap between two dates is one of the most reused calculations in ordinary life, well beyond curiosity about an event.
- Shipping and delivery windows — service promises are frequently quoted as a fixed number of days from an order date, and the count has to match what the carrier actually means.
- Contract notice periods — resignation, lease termination and cancellation clauses commonly specify a minimum number of days between giving notice and the change taking effect.
- Subscription and billing cycles — free trials and pro-rated charges are calculated from the exact number of days a customer used a service, not a rounded month.
- Pregnancy and medical timelines — gestational age and appointment scheduling both run on exact day counts from a reference date.
- Interest and day-count conventions — bonds and loans accrue interest under rules — actual calendar days, or a standardized 30-day month — that all begin with a date difference.
The ISO week number and the day of the year
The "day of the year" figure counts from 1 January as day 1 through to 365 or 366 on 31 December, and shows up in manufacturing date codes, agricultural planning calendars, and payroll or production systems that run on a running day count rather than a month-and-day one.
The ISO week number comes from a related but separate standard, ISO 8601, which divides the year into 52 or 53 numbered weeks, each running Monday to Sunday. Because week 1 is anchored to whichever week contains the year’s first Thursday rather than to 1 January itself, the last few days of December or the first few days of January sometimes belong to a week numbered in the other calendar year — a detail that trips up anyone building a week-based schedule by hand.
Counting inclusively or exclusively
Whether the end date itself counts as a full day depends entirely on what is being measured, and getting it wrong is a classic off-by-one mistake. A hotel booking from Monday to Thursday is three nights, not four, because the departure day is not an overnight stay. A rental agreement running "from the 1st to the 31st", by contrast, usually means all thirty-one days are covered, including the last one.
This calculator defaults to the exclusive count — the plain elapsed gap — and lets you switch on an inclusive count for cases like bookings, project timelines and notice periods, where both endpoints genuinely belong to the span.
What this assumes, and where it stops
Assumptions
- Dates are calendar dates with no time component.
- Working days are Monday to Friday.
Limitations
- Public holidays are not excluded from the working-day count — they vary by country and region.
- Time zones and times of day are not modelled. For sub-day precision you need a timestamp difference, not a date difference.
Common questions
Should I include the end date?
It depends what you are counting. For "how many days until", leave it off — the answer is elapsed days. For "how many days does this booking cover", turn it on, because both the first and last day count. The difference is always exactly one day.
Why does the month count not match days ÷ 30?
Because months are not 30 days. A span from 31 January to 28 February is one calendar month but only 28 days; from 1 July to 1 September is two months but 62 days. The calendar span and the day count answer different questions.
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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