This is a weird one. If you look at your calendar in a browser in a Parallels Windows virtual machine the times will be off by one hour. This is true in both Firefox and IE 6.
Linux virtual machines are unaffected. Calendar meeting times are correct in both Firefox and Seamonkey.
Tested on Parallels for Mac, don't know if other host OSes are affected.
Note that this is a client-side thing, the server is not running under parallels.
It's almost certainly a problem with Parallels and not with Zimbra.
Have been using Zimbra Open Source for six months now for a 6-9 person office, and we're pretty happy with it. The client experience is pretty good, and the web client is excellent.
Moving it from Fedora to a Debian chroot in Gentoo was a royal pain, but I found all the answers I needed by lurking on these forums.