Date Calculator

The number of days between two dates seems like a question with one answer. It has two, and which one is correct depends on a convention nobody states.

Inclusive or exclusive

From 1 April to 10 April is either nine days or ten, depending on whether the first day counts.

Exclusive of the start date gives 9. This is the default in most software, including this calculator, and it is what "the interval between" ordinarily means. It also gives the sensible result that the interval from a date to itself is zero.

Inclusive of both ends gives 10. This is what people mean by "how many days will I be there" — a hotel stay from the 1st to the 10th is ten days of presence and nine nights.

Neither is right in general. What matters is saying which you are using, particularly in a contract, a notice period or a leave application, where a difference of one day can be the difference between compliance and breach.

Indian statutory periods generally follow the General Clauses Act: the day of the event is excluded and the last day is included. A contract can say otherwise, and where it does, the contract governs.

Total days is the safest number

Where a calculation carries a consequence, count days rather than years and months.

The total number of days between two dates has exactly one correct answer. There is no month length to borrow from, no convention about what a month means, and no disagreement between implementations. Interest accrual, notice periods, warranty terms and medical dosing all work this way for that reason.

Years, months and days is a human summary of the same interval, and summaries involve choices. Two tools reporting different month counts usually agree on the day count and differ only in how they present it, which is the fastest way to diagnose a disagreement.

Business days

A business day count excludes weekends and holidays, which means it cannot be computed without a holiday list — and India does not have one list.

Weekends vary. Most private offices close Saturday and Sunday. Banks in India close on the second and fourth Saturdays, and are open on the first, third and fifth. Government offices vary by state.

Holidays vary. Gazetted holidays are set centrally, but each state adds its own, and several — Ganesh Chaturthi, Onam, Bihu, Pongal — are holidays in some states and ordinary working days in others. Restricted holidays are optional per employee.

Financial markets have their own calendar, which is not the same as the banking calendar.

The practical consequence is that a stated period of "ten working days" means different elapsed time in Mumbai and in Chennai. Where it matters, specify calendar days instead, or name the holiday calendar being used.

Most Indian statutes provide that a deadline falling on a holiday moves to the next working day. A few, including certain tax deadlines, do not — which is worth checking rather than assuming.

Day-count conventions in finance

Interest calculations use explicit conventions, and they produce different amounts on the same principal and rate.

On ₹10,00,000 at 9% for a period of 91 actual days, actual/365 gives ₹22,438 and actual/360 gives ₹22,750 — ₹312 apart on one quarter, purely from the denominator. Over a year of rolling quarters the gap is meaningful, which is why the convention is stated in the loan document rather than left to be assumed.

Actual/360 always produces slightly more interest than actual/365, which is why lenders prefer it and why it is worth noticing in a term sheet.

Leap years

A year is a leap year if it is divisible by 4, except centuries, except centuries divisible by 400. 1900 was not a leap year; 2000 was; 2100 will not be.

The rule exists because the tropical year is about 365.2422 days. Adding a day every four years overcorrects slightly, and the century exceptions remove the excess. The Gregorian calendar's residual error is roughly one day in 3,200 years.

Any date calculation spanning 29 February must count it, which is why manual estimates using "365 days a year" drift over long spans — over a 40-year period, ten days.

Week numbers, and why systems disagree

Two conventions are in common use and they can differ by a week.

ISO 8601, used across Europe and in most software, defines week 1 as the week containing the first Thursday of the year, with weeks starting on Monday. A year has 52 or 53 ISO weeks, and 1 January can fall in week 52 or 53 of the previous year.

The US convention starts weeks on Sunday and defines week 1 as the one containing 1 January.

Reports generated in two systems with different conventions will disagree about which week a date belongs to, near the year boundary always and elsewhere sometimes. Where week numbers are shared between organisations, naming the convention avoids a class of reconciliation problem that is otherwise very hard to see.

Practical cases

Notice periods. A "30 days notice" served on 1 April ends on 30 April under exclusive counting. Whether you must work the last day is a matter for the contract.

Age for exams and admissions. Almost always measured against a fixed cut-off date, not against the exam date, so someone who crosses the limit in September is still eligible for an exam with an August cut-off.

Limitation periods. Time to file a legal claim runs in calendar years from the cause of action, and the General Clauses Act's exclusion of the first day applies.

Warranty and return windows. Usually counted from the date of delivery rather than the date of purchase, which for online orders are different dates.

Project planning. Working days, which means the holiday calendar has to be agreed before a schedule means anything.

Adding a duration to a date

The reverse operation — a date plus a period — has its own edge case, and it is the one that breaks naive implementations.

What is one month after 31 January? There is no 31 February. The near-universal convention is to clamp to the last day of the target month, giving 28 or 29 February, and that is what this calculator does.

The consequence is that adding months is not reversible. One month after 31 January is 28 February, and one month after 28 February is 28 March — not 31 January. Anyone generating a schedule by repeatedly adding one month to the previous result will drift, and every date after the first short month will be wrong.

The fix is to always add to the original date: date + 1 month, date + 2 months, date + 3 months, rather than iterating. Loan schedules, subscription renewals and EMI dates are all generated this way for exactly that reason.

Adding years has the same issue only on 29 February, which becomes 28 February in a common year.

Dates in spreadsheets and code

Two quirks account for most date bugs outside this page.

Spreadsheets store dates as numbers. Subtracting one cell from another gives a count of days directly, which is convenient — and formatting that result as a date rather than a number produces a nonsensical day in January 1900. Excel also believes 1900 was a leap year, a deliberate bug retained for compatibility with a 1980s competitor.

Programming languages count months from zero. In JavaScript, month 0 is January and month 11 is December, which produces off-by-one-month errors that survive testing because they look plausible.

Both are reasons to prefer an unambiguous ISO format — 2026-09-06 — whenever a date crosses a boundary between systems or people. It sorts correctly as text, carries no regional ambiguity between day-first and month-first, and cannot be misread.

A number of days from a date

"What is the date 90 days from now?" is a different question from the gap between two dates, and it has its own mode above. Starting from Thursday, 1 January 2026, the common offsets land here:

Counting in working days skips Saturdays and Sundays, and the start date is day zero: five working days from Thursday, 1 January 2026 is Thursday, 8 January 2026. Public holidays are not skipped, so a deadline that spans one needs a day added by hand.

Work experience between two dates

An experience certificate runs from the date of joining to the last working day, and both of those days were worked, so the end date is counted. From 1 July 2019 to a last day of 30 June 2024, counting the end date gives 5 years, or 1,827 days. Leaving it out gives 4 years, 11 months and 29 days — one day short of five full years, and the difference between "five years' experience" and "four years and eleven months" on an application form.

What this calculator assumes

• The start date is excluded and the end date included, so a date to itself is zero days.
• Dates are treated as local calendar dates with no time component, so time zones do not shift the result.
• The proleptic Gregorian calendar is used throughout, so dates before 1582 are counted as though its rules had always applied.
• Business day counts exclude Saturdays and Sundays; regional and religious holidays are not applied, because there is no single national list.
• Where the years-months-days breakdown is shown, months are anchored to real calendar months rather than assumed to be 30 days.