Date calculator

Almost always because of whether the first day counts. From Monday to Wednesday is either two days or three, depending on whether you're measuring the gap or counting the days involved.

Both are correct for different questions. This one gives the gap, which is the usual answer: Monday to Wednesday is 2. If you need the count of days including both ends, add one.

Which convention you need

Age and anniversaries use the gap. You're 30 on the day thirty years have elapsed, not thirty-one.

Notice periods and deadlines vary by contract and jurisdiction, and this is where the difference bites. "Fourteen days' notice" given on the 1st might expire on the 14th or the 15th. Contracts often say "clear days", which means excluding both the day it's given and the day it expires. Read the wording; the arithmetic follows from it.

Hotel and rental nights count nights, not days. Arriving Friday and leaving Sunday is two nights and touches three days.

Court and legal deadlines frequently have their own rules, including excluding weekends and public holidays, and rolling forward if the last day falls on one. Never take a general calculator's answer for a filing deadline.

Where months get slippery

Days are constant, months aren't. Adding one month to 31 January has no clean answer, because 31 February doesn't exist.

The near-universal convention, and what happens here, is to clamp to the last valid day, so 31 January plus one month is 28 February, or 29 in a leap year.

That makes month arithmetic non-reversible. 31 January plus one month is 28 February, and 28 February minus one month is 28 January, not the 31st you started from. This surprises people setting up monthly schedules, and it's why a subscription started on the 31st can drift to the 28th and stay there in some systems.

If a date must land on the same day every month, pick something at or below the 28th.

Leap years

Every year divisible by 4, except centuries, except centuries divisible by 400. So 1900 wasn't a leap year and 2000 was. It matters for anything spanning those boundaries, and it's a classic source of one-day errors in older software.

29 February birthdays are handled differently by different systems, some treating the legal date as 28 February and some as 1 March. There's no single right answer and countries genuinely differ.

Working days

Counting only Monday to Friday is a different calculation, and the answer depends on which country's public holidays apply. A "ten working day" turnaround means different real dates in Lagos, London and New York, and different again around Christmas or Eid.

If a deadline is quoted in working days, get the specific dates confirmed rather than calculating them yourself. This is one of the most common causes of missed handovers.

Time zones

Dates without times are ambiguous across time zones. A deadline of "the 15th" is a different moment in Lagos than in Los Angeles, and international contracts that don't specify a zone create a genuine gap of up to a day.

If a date matters legally, it should say the time and the zone. If it doesn't, assume the other party's zone and be early.

What this is for

Quick answers to ordinary questions: how long until something, how old something is, what date is 90 days out. It's arithmetic on a calendar, and it doesn't know about holidays, contracts or jurisdictions.

For anything where being a day out has consequences, treat the result as a starting point and confirm it against the rules that actually govern it.

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