fand ich die Lösung:
<p:outputLabel for="startDate" value="#{msg.StartDate}" />
<p:calendar id="startDate" value="#{employeeBean.employee.startDate}" required="true" pattern="dd-MM-yyyy" language="#{msg.CalendarLanguage}" mask="true"
requiredMessage="#{msg.required}">
<p:ajax event="dateSelect" update="endDate" />
</p:calendar>
<p:message for="startDate" />
<p:outputLabel for="endDate" value="#{msg.EndDate}" />
<p:calendar id="endDate" mindate="#{employeeBean.employee.startDate}" value="#{employeeBean.employee.endDate}" required="false" pattern="dd-MM-yyyy"
language="#{msg.CalendarLanguage}" mask="true">
</p:calendar>
<p:message for="endDate" />