Is “in [some period]” different from “within [some period]”?
The title states it all: When an author says "within 7 days", does the author mean include the 7th day? Or, does it mean up to but not including the 7th day?
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I think within seven days includes the seventh day. I take it to mean "by the end of the seventh day."
For example, the American Red Cross blood donation website has this line:
• Donated platelets must be used within five days of collection.
I take that to mean the platelets can be used on Days 1, 2, 3, 4, and 5. But on Day 6, they have to be discarded.
(The fact that you were prompted to ask your question, however, is why I always state specifically that I want something returned to me by the end of the business day on a certain date.)
I believe the answer is that ordinary language is simply not precise enough to say, without a little extra acrobatics. You could ask the same question about other time limits, such as "by Friday" - does this mean that you have until midnight on Friday? I think there is a tendency (here's the acrobatics) to use the next smaller unit when precision is necessary, for example "48 hours". So, if you wanted to specify exactly one week, you could say "168 hours".
Whenever I have a question like this I try to break it down into a smaller case and then extrapolate. If I ask to have something down "within a day" I mean 24 hours from now - today is day 0 and tomorrow is day 1, the day that it is due. So "within 7 days" means it is due on day 7, counting today as day 0.
It's ambiguous. I'd say "within 7 days" SHOULD mean within 7 x 24 hours from now, so if at noon on Monday I'm told "this is due within 7 days" that would mean by noon of the following Monday. But people often take today to be the first day of the count, so if on Monday someone says "within 3 days" they are thinking day 1=today, Monday; day 2=Tuesday, day 3=Wednesday.
There's also the perennial question of whether the last day ends on the multiple of 24 hours from the time when the deadline was given, if it means midnight of that day, or closing time of that day, or what.
And does "7 days" mean 7 calendar days, or 7 business days?