Check the date.
js code or the code on our MiniCal where we prepopulate a Zimbra appointment form. Both of these allow the user to first verify/check the inputs before creating the new event. If you want to just add a new event you might want to try the REST interface and post the data directly to that.
Here's some info on how to POST new data to Zimbra.
http://www.zimbra.com/blog/archives/..._via_re_1.html