Countdown between dates: when to include the start and end day
Avoid an off-by-one error when counting a deadline, hotel nights, delivery window or event countdown.
How to calculate it
A date difference normally counts elapsed midnight boundaries: end date minus start date. This excludes the start date and gives zero if both dates are the same. An inclusive list of calendar dates usually adds one if the interval is valid. Hotel nights use the difference, while days of a multi-day event may include both endpoints. Decide the question before applying a formula, and never apply the inclusive adjustment to time durations without checking the times of day.
Worked example
From 10 October to 13 October, the date difference is three days: the nights of the 10th, 11th and 12th. If a workshop runs on the 10th, 11th, 12th and 13th, it occupies four calendar dates. An order promised within three business days is a different question: weekends and local holidays may need excluding, and a seller may define the start time after a cutoff. Label the unit and convention next to the answer.
What can change the result
Time zones can move an instant to a different local date. A deadline at 23:59 in one place can already be the next day elsewhere. Daylight-saving changes can make elapsed hours unequal to 24 times the calendar-day difference. Leap days are handled by calendar arithmetic; adding a fixed number of seconds is not always a substitute.
Use the calculators and keep reading
Use the date-difference calculator for elapsed days and the date-add calculator for a target date. State whether you include both endpoints. For work deadlines, use the business-days calculator and consult the linked inclusive-counting guide.
Try the date difference, date add calculator and read this related article for the context and assumptions.