That bug has been opened on https://launchpad.net/distros/ubuntu/+source/evolution/+bug/58210

"Version: 2.6.1-0ubuntu7 / Dapper
Problem: When adding a task in the calendar I get a dialog "invalid time value" when clicking "save". I have indeed chosen a time in the drop-down box (for instance 09:30). After clicking OK in the "invalid time value" dialog it reappears together with another dialog with the message: "Validation error: the starttime is wrong" (translated from the error message that is in norwegian).

A google search:

http://www.mail-archive.com/evolution-list@gnome.org/msg04216.html

In my case the time values always show up as hh:mm and changing the format to anything I can think of does not help (hh.mm, hh;mm, etc).

I have tried with a fresh $HOME/.evolution without sucess (it was a totally new user account with no custom configuration).

http://librarian.launchpad.net/4043207/invalid_time_value1.jpg
Screenshot of the first dialog with the "invalid time value" message
...
> Thanks for your bug. What locale do you use?
...
erlingre@skarphedin:~$ locale
LANG=nn_NO.UTF-8
LANGUAGE=nn_NO:nn:no_NO:no:nb_NO:nb:en_GB:en
LC_CTYPE="nn_NO.UTF-8"
LC_NUMERIC="nn_NO.UTF-8"
LC_TIME="nn_NO.UTF-8"
LC_COLLATE="nn_NO.UTF-8"
LC_MONETARY="nn_NO.UTF-8"
LC_MESSAGES="nn_NO.UTF-8"
LC_PAPER="nn_NO.UTF-8"
LC_NAME="nn_NO.UTF-8"
LC_ADDRESS="nn_NO.UTF-8"
LC_TELEPHONE="nn_NO.UTF-8"
LC_MEASUREMENT="nn_NO.UTF-8"
LC_IDENTIFICATION="nn_NO.UTF-8"
LC_ALL="

The same issue happens with GNOME 2.16.0 with a nn_NO locale. Might be a translation issue, is the program supposed to depend on translations for that?