This word will be ancient history to anyone who has worked with television or radio scheduling, because daypart has been part of scheduling jargon for probably a quarter of a century or more. What interested me in the word was reading an article on Internet advertising and seeing daypart used without explanation, as though the reader should, after all, be familiar with such a term. It wasn't hard to figure it out from the context, but I wanted to see if the word was being used casually in non-industry publications. What I found was that, yes, the media quite often used the word without so much as a how-do-you-do. However, the majority of cites fell into one of two camps — using the word with an explanation or an example (as in the first citation), or including the word as part of a quotation from someone in the industry, as seen in the earliest citation.