I type at the top of my zul:
<?xel-method prefix="y" name="isDateFirstDayOfAMonth" class="com.myproject.MyClass"
signature="boolean myMethod(Date)"?>

The parser cannot find the Date class but the error message says :
Caused by: org.zkoss.zk.ui.UiException: file:/C:/Proje...sub/valorisationGrille.zul:6:52: Class not found: com.myproject.MyClass

When in fact it broke off at org.zkoss.lang.Classes.getMethodBySignature() when trying to get the class Date from getClassOfSignature(resolver, clsnm) art the line 653.

The name of the class causing the ClassNotFoundException is in the DetailMessage attribut, so easy to add the correct class name causing the exception.