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Is it incorrect to say "Please do this before Tuesday"?

Is there a difference between that and "Please do this by Tuesday"?

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I object to this question being a duplicate. The linked question addresses the meaning of "by Monday". My question is whether it is correct to say "before Monday" at all, and the meaning of it. – R C Mar 5 '13 at 19:18
(The reason I'm asking is that I've been told that "before date" is incorrect and one should say "by date".) – R C Mar 5 '13 at 19:25
There have been several other questions of this general type (What does “by spring 2013” imply?, for example). Note that your question doesn't actually say you're wondering whether "before Monday" is valid at all (it is, of course). Also note that although both answers here assert an absolute distinction between by/before, the reality (as covered by several answers to other questions) is that people frequently don't distinguish that precisely, so you may end up being misled. – FumbleFingers Mar 5 '13 at 21:04
@FumbleFingers: it says that in the very first sentence, unless I'm missing something. Regarding the question you link to, again, it doesn't mention "before date" at all, which is exactly the point of this question. – R C Mar 5 '13 at 21:17
up vote 9 down vote accepted

There is a difference, yes.

If you want the job to be ready on Tuesday morning, you would say

Please do this before Tuesday

If you need it to be done either before or during Tuesday, you would say

Please do this by Tuesday

In other words, using by is inclusive, it means do this on any day up to and including the day specified. Using before is non inclusive, it means that I expect it to be done when I arrive on Tuesday morning.

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If you want to be precise and want it done literally before a certain time, then "before" is the the word to use. If you want it done on or before the specified day or time, the "by" is the right word.

A couple of other expressions that are used (especially in business) are "not later than" (abbreviated NLT)) and "by close of business" (abbreviated COB).

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