Thanks G.edwin,
I got it working this way, the problem I still had was in web mode in combination with tomcat. Since it is not easy to control the Classloader in tomcat the gwt-servlet.jar was always loaded prior to my jar named owhatever.jar. I figured out that tomcats classloader order is alway alphabetic so that g*.jar is always loaded prior to o*.jar. So the com.google.gwt.user.client.rpc.core.java.util.Date_CustomFieldSerializer class was always token from gwt-servlet.jar. To Solve this one could either rename the jars to be loaded in the desired order which I did not like since this might be implementation dependent or to include the code from gwt-servlet.jar except of the Date_CustomFieldSerializer class within my owhatever.jar which I finally did!