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Which is better, and what is the difference?

What is the best time to call back?

When is the best time to call back?

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Either, but I'm more concerned about callback. I can see no reason for making it one word.

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I can see a reason for not making callback a single word - in this context, it's simply incorrect. – FumbleFingers Jun 4 '12 at 16:45
The word callback exists as a noun in programming, but I do not support verbing it. – chaos Jun 4 '12 at 16:45
yeah my mistake, thank you though for correcting. I just want to know which is more convenient to use. When or what? – post Jun 4 '12 at 16:49
@post: There really isn't any significant difference. You'll find about the same number of written instances in Google Books for both "What" and "When is the best time to...". And that's still the case if you change is to 's, showing that neither is any more or less "formal" than the other. Not every different choice of words has significance. – FumbleFingers Jun 4 '12 at 16:55

Either will be understood, but "What is the best time to call back?" is more commonly used.

At a technical level, using "what" is modeling the answer as a generalized conceptual entity, while using "when" is specifically modeling it as a chronological time. Nobody is ever thinking about this distinction when using either phrase, however.

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I'm not sure I agree that what is more common. Do you have a source? I would use "when" if asking about a time. I might say "What time is best to call back?" But using the OP's sentence, I'd say "When is the best time to call back?" – JLG Jun 4 '12 at 16:55
-1 because I'm feeling a bit tetchy today. I find no evidence in Google Books to suggest that either what or when more often precedes is the best time to.... They are pretty much equal in terms of meaning, prevalence, and "degree of formality", so I dispute your statement about "is more commonly used". – FumbleFingers Jun 4 '12 at 17:33
@FumbleFingers: Try Google Web Search. 37.2MM vs. 29.8MM. – chaos Jun 4 '12 at 18:11
@chaos: Sorry, but I think Google's "About nnnn results" there are "guesstimates", which reflect nothing more than the fact that "what" is more common on the Net than "when" (by a factor of 4:3). In Google Books, "what's/when's the best time to both give about 6.5K results, which I think means any "preference" is marginal, to say the least. – FumbleFingers Jun 4 '12 at 20:09

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