This behavior occurs on not-required Dates too, so I posted this issue according to
JSF-20 and
JSF-25
fixed this problem by changing templates.jsf.configuration.validation.xml.vsl line 22:
#if $field.validationRequired && $actionParameters.contains($field))
to
#if (!$field.readOnly && $field.validationRequired && $actionParameters.contains($field))
suggestion: think about $field.validationRequired returning false for readonly values