Client OS: Windows XP sp2 English edition & IE6
go to "Control Panel" -> "Regional and language Options"
in "Standards and formats" select "Dutch (Belgium)" -> Apply -> OK
start IE6
http://your.zimbra.url
login
press "Ctrl + r" (refresh) !!!!!!! you must do this to replicate
go to the calendar
select "Month" view
go to "August"
right click on a day -> "New Appointment"
fill in the "subject" -> click "Save".
Now you will see that there is no appointment for that day, go to "December" and you can see the appointment at the same day and time.