Adding a Manual Event on the Scheduled Events page and the time entry/dropdown lists seem to use the 12 hour clock. Everything else on that page uses the 24 hour clock. Ugh. Weird and very off-putting.
Seems to work even if the accompanying documentation is obscure or incomplete. Possibly it would be nice if the top item in the pull-down was the next step in the the list after now (and wrap around after 24:00) but I can't get excited.