Apparently wsimport used XMLGregorianCalender java attributes which caused the SchemaValidation problem.
I solved it using a jaxb binding file so Calender java objects are used instead.
binding file "bindings.xml":
<?xml version="1.0" encoding="UTF-8" ?>
<jaxb:bindings
version="2.0"
xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
xmlns:xs="http://www.w3.org/2001/XMLSchema">
<jaxb:globalBindings>
<jaxb:javaType name="java.util.Calendar" xmlType="xs:dateTime"
parseMethod="javax.xml.bind.DatatypeConverter.parseDateTime"
printMethod="javax.xml.bind.DatatypeConverter.printDateTime"/>
<jaxb:javaType name="java.util.Calendar" xmlType="xs:time"
parseMethod="javax.xml.bind.DatatypeConverter.parseTime"
printMethod="javax.xml.bind.DatatypeConverter.printTime"/>
<jaxb:javaType name="java.util.Calendar" xmlType="xs:date"
parseMethod="javax.xml.bind.DatatypeConverter.parseDate"
printMethod="javax.xml.bind.DatatypeConverter.printDate"/>
<jaxb:serializable uid="1"/>
</jaxb:globalBindings>
</jaxb:bindings>
and added "-b bindings.xml" to the wsimport command:
wsimport -keep -Xnocompile -b bindings.xml some.wsdl
Extra adapter.java files are generated for the conversion, don't forget to use them in your java project.
Hope someone else will find this useful as well!