Seven Days or Eight? Inclusive and Exclusive Day Counting

The gap and the count

Seven is the gap: the number of nights, the number of steps from one date to the other, the number you add to the first to reach the second. It is what subtraction gives you, and it is what a computer gives you by default.

Eight is the count: how many dates are involved, the 1st and the 8th included. It is what you want when something is true on both end days: days you are in a hotel, days a permit is valid, days an employee is on site.

Each answer is right for its own question. A hotel that charges by the night charges for seven; a visa that is valid on both endpoint days covers eight. The mistake is giving the number without saying which one it is. The date calculator shows the gap by default and the count when you tick a checkbox, and it names the reading it used under the answer.

What the common phrases usually mean

English has rough conventions for these phrases. No document is bound by them:

• "Between X and Y": usually the gap. The two endpoints mark the edges and are not counted.
• "From X to Y": ambiguous, and a frequent cause of real disputes. A train that runs from Monday to Friday runs on Friday; a discount from Monday to Friday leaves Friday in doubt.
• "X through Y": in American usage, the count. "Through" includes the far end, which is why careful drafters use it.
• "Within N days of X": the gap, measured forward from X, with X itself excluded.
• "For N days": the count. A three-day event occupies three dates.

Where a document defines its own terms, the definition governs. Where it does not, readers will assume these conventions, so match them on purpose when you write one.

The day of the act is not counted

One rule is nearly universal across legal and commercial drafting: when a period runs from an event, the day of the event is excluded and counting starts the next day. A notice served on the 3rd with a ten-day period runs from the 4th. The reason is practical. The event can happen at any hour, and counting a part day as a whole one would make the deadline depend on the time of day the notice arrived.

The other half of the rule catches people out: the last day is counted, in full. A ten-day period beginning on the 4th ends at the close of the 13th, not the 12th.

Why an off-by-one is easy to miss

An off-by-one in a day count is hard to notice. The answer is plausible, close and wrong, and nothing about it looks unusual. It does the most harm on spans of a few days, where one day is the largest share of the total and the cost is concrete: a missed filing, a double-charged night, a permit that expired yesterday.

Whenever a count matters, state the convention with the number. "Eight days, counting both the 1st and the 8th" cannot be misread; "eight days" can.