A clock is a template that stations and networks use to program on-air: when to start and end a show, where to place breaks for local announcements and underwriting, when newscasts should be inserted, etc.
The more you know about clocks and how they're used, the more strategic you can be about producing pieces that will have practical appeal to stations and networks. For example, even though a 31-minute program might sound great, it is not a natural fit within a clock that shows most programs would end at 29 minutes. Or, an 11-minute story would be hard to wedge into a Morning Edition clock with clearly defined segments.
Below are a variety of hour-based clocks used by NPR and, by extension, stations and many program producers. If you produce an hour-long program, you'll want to take a close look at where to schedule your "news hole" (where a newscast gets inserted), cutaways for local station breaks, and when to end the program.