The trick is to format the date correctly before sending it to the datepicker, and let datepicker what format it is supposed to be usinging. 

<ww:form name="form" action="to_infinity_and_beyond" method="post">
     <ww:date name="%{new java.util.Date()}" format="dd-MM-yyyy hh:mm" id="date"/>
     <ww:datepicker value="%{#date}" showstime="%{true}" format="%d-%m-%Y %H:%M"/>
</ww:form>

contributed by Philip Luppens