I’ve just started working on moving my site to 1.10, so can’t help much... but here goes....
> Pro: You can now choose your time zone so times are correct for your time zone and not the server (which may be somewhere else). This is HUGE -thanks!
> Con: If you live in a country that uses AM and PM you may be out of luck. In 1.9 and below there was no way to choose 12hr/AM/PM but you could edit the Calendar and Add Event template files and force them to use AM/PM and 12hr time. In 1.10 the template’s changed and it doesn’t appear you can make that same template change. There is nothing in Features/General or Wiki (or Calendar) Admin that fixes this. The Calendar in 1.10 respects the date but not the time setting in Admin/General/Long and Short date & time format settings. Unless I am missing something if you use 12hr time you may have to stop using the calendar.
You need to edit the calendarlib.php library (look at/near line 193). It appears that the time is hardcoded as %H:%M. You can change it there.