Not seen (in, for) a certain period of time?

[Mwita Chacha]

As part of my campaign to get the hang of good English, I listen to the BBC World Service as well as reading the BBC official news website on daily basis. The tangible outcome is appreciated. But that doesn't prevent me from quoting a BBC sentence and asking for clarification on its grammar if I happen to be doubtful about it. This is the reason I now and then do that on the Forum.
There is a trial in progress in China against Mr. Bo Xilai, a disgraced political leader charged with embezzlement, corruption, and abuse of power. I don't have much interest in the hearing, but certainly I would like to know the grounds for the use of preposition ''in'' in this sentence by a BBC China correspondent in his radio report of the case: ''Bo Xilai has never been seen in public in 18 months.''
If I were the correspondent, I would have used preposition ''for'' instead. I speculate you certainly would have done the same thing.

[Joe Carillo]

Although it’s acceptable in another sense to use the preposition “for,” I think the BBC China correspondent’s use of “in” is more appropriate both grammatically and semantically in this sentence you presented: “Bo Xilai has never been seen in public in 18 months.” It’s evident that the BBC correspondent had chosen “in” to indicate “18 months” as a limit, not as a durarion. To indicate a limit is one of the many uses of the preposition “in,” which in this particular case is to indicate inclusion, location, or position within physical or time limits, as in “The flood victim’s body was found in the river,” “The diabetes patient was injected in the leg,” and “We thoroughly enjoyed ourselves in the summer.” In this usage, “in” indicates a point or limiting range in space or time.

In contrast, one of the many uses of the preposition “for” is to indicate duration of time or extent of space, as in “They vacationed in Tuscany for two weeks last summer” and “They drove for miles in the open desert.” Here, the sense is on the length of time or distance or area covered.  It would then be logically questionable to say that “Bo Xilai has never been seen in public for 18 months,” for that would mean that the BBC correspondent is making the audacious, uncalled for, and highly improbable claim that although everybody was watching, absolutely nobody had seen Bo Xilai in a public gathering during that period. There’s just no way the BBC China correspondent can be absolutely sure of that!  Indeed, I would think that using “for” in such a sentence would be logical and justifiable only if the verb denotes an action that the subject himself is fully in control of, as in “The beleaguered starlet says that after figuring in the sex-video scandal, she has not shown herself in public for two weeks now.” She knows what she did for a full two weeks and she’s telling us about it.

In sum, to say from a journalistic standpoint that “Bo Xilai has never been seen in public in 18 months”—time as limit or boundary—is likely to be truthful and more probable than “Bo Xilai has never been seen in public for 18 months”—time as duration. The big difference is in the sense of the two statements.