If you guys could point me somewhere I can look, that'd be great.
The problem is local to one site inside a site collection.
Recurring events worked fine; one user had an event marked every other Monday and Friday. He created a multiday event, and now recurring events appear as one single event with an end date exactly 998 recurrence-lengths forward (e.g. an event created 5 Aug, 2013 that recurs once weekly would end 20 Sept, 2032, 998 weeks later). No workflows or custom code are associated with this calendar, and new calendars on this same site experience the issue while calendars on other sites do not.
A similar problem is described in Recurring Events in SharePoint 2010 - Incorrect "Duration" where it is suggested that it may be a SP2010 bug.
If I can provide more info, please let me know.