I’ve used Plazes on and off for maybe a year. I can see their reluctance to call themselves a calendar, a market segment filled with competitors and existing notions of what calendars can and should do. Instead they focus their platform around places. Recent feature updates there, though, refocus the site around activity, not place, and thus as a calendar, but only partially. Collaborative calendars such as Eventful and collaboratively-filtered event listings such as Last.fm do a better job.
Dropping support for semantics such as activity finish seems like a bad idea. Rather than pioneer a new format for now old and time-tested expressions, why not support existing calendar standards? Plazes now provides beta iCalendar feeds for past activity tracking. Extended full compatibility support for iCalendar and its XML equivalent hCalendar, with time-tested notions of time, place, activity and activity end, with roots in vCalendar, then makes Plazes and its database more useful, flexible, and valuable. Why must Plazes reinvent the calendar?