From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.8) Gecko/20060103 Firefox/1.0+

Description of problem:
see "Steps to Reproduce"

Version-Release number of selected component (if applicable):
evolution-2.5.4-10

How reproducible:
Always

Steps to Reproduce:
1. start evolution calendar with LANG=tr_TR.UTF-8
2. using the date selection widget, try to create a new appointment on 5th 
of January. notice the appointment created on 1st of May.
3. try creating another appointment on 20th January (which can also be done
by clicking "today" on the date selection widget today). 
-> "Invalid Date Value" error
  

Actual Results:  either the appointment is created on the wrong date
or an error dialog pops.

Additional info:

i also filed this as a comment to http://bugzilla.gnome.org/show_bug.cgi?id=250754
but thought i should mention it here too