Now I've got the by date view working, previous and next become more pressing.

I need to make a choice about semantics: Does previous/next always refer to entries, or on (e.g.) a day view, does it mean the previous/next day, even if there aren't any entries.

Ignore that, it's clearly about entries. Probably once I add in other data sources, there's going to be entries every day anyway, but I might as well skip empty days (and stop at the date of the earliest entry). (A reader who decodes the URL format is welcome to load a blank page, but if I add auto links to the infinite past, some stupid spider will blindly follow them.)(serves it right)