I've got a couple of (possibly related) issues with the Calendar app in Mountain Lion which I can live with but looks messy:
My preference is the 12-hour clock with am/pm appended, and my date/time system prefs are set as such.
However, Calendar does the following:
1. "On the hour" events (eg 9:00am, 1:00pm) simply display as 09 or 13, even though other "off hour" events display correctly (eg 12:15pm).
2. Start and end times sometimes mix 12 and 24 hour time, even when the start time is after noon. e.g. I have an event marked as 7.45pm to 22 instead of 7.45pm to 10:00pm. (This probably is related somehow as it happens only for 'on the hour' events as problem 1).
I can't find any other place to set preferences - only whether the time is shown or not.
Any ideas how to fix this, even if it's a hack? All my system software is up-to-date. Interestingly, switching the system pref to 24 hour clock doesn't seem to update the Calendar (even after restart) so is Calendar getting this preference from elsewhere? It is synced with Google Calendar but I don't see how any settings there will affect what to me seems to be a display issue.