A calendar application is a bit different, since the whole subject of the application is time, time zones, etc. Still I'd store all in UTC along with the user's time zone (you'd have to store the time zone anyway with localized time too).
Storing local time is preferable because it makes working with times across DST boundaries a non-existent problem and because users frequently set their own timezone incorrectly.
I've done previous projects where I've stored UTC dates or unix timestamps for this situation and it was a hassle to deal with. You really need to consider the nature of the data -- for straight forward timestamps (like the date of a post) UTC makes the most sense. For user-entered dates, I think local time is much more appropriate and a lot simpler.
