Days from today

The off-by-one problem

Ask how many days there are between the 1st and the 8th and two reasonable people will give you 7 and 8. Both are right, for different questions. Seven is the number of days elapsed — the gap. Eight is the number of days involved, counting both endpoints. Deadlines, notice periods and hotel bookings each pick a different convention, and mixing them up is the single most common date-maths error.

This calculator reports elapsed days: the difference between the two dates. If you need an inclusive count — for a holiday itinerary, say, or a jury summons — add one.

Why you cannot just multiply months

Counting in whole days avoids all of it, which is why legal and contractual deadlines are almost always specified in days rather than months.

• Months are 28, 29, 30 or 31 days, so 'three months' has no fixed length.
• Leap years add 29 February every year divisible by four, except century years not divisible by 400. 2000 was a leap year; 1900 was not; 2100 will not be.
• Daylight saving shifts the clock but not the calendar — a day count is unaffected even though one day that year is 23 hours long and another is 25.
• Adding months to the 31st is ambiguous: one month after 31 January is usually treated as 28 or 29 February, and the day-of-month is lost.

Working days versus calendar days

Contracts, refunds and notice periods often say 'working days'. That excludes weekends and public holidays, and public holidays differ by country and sometimes by region. A ten-working-day turnaround can be anywhere from fourteen to eighteen calendar days depending on where the bank holidays fall.

What people use this for

• Counting down to a holiday, a wedding, an exam or Christmas.
• Checking a contractual notice period or a returns window before it expires.
• Tracking a pregnancy due date, a project milestone or a probation period.
• Working out an anniversary in days, or how many days old someone is.
• Adding or subtracting a fixed number of days from today to find a target date.