code: public String getFormValueAsString() {
  
          Date value = getValue();
          if (value == null) {
              return "";
          } else if (allowInvalidValue() && value.equals(INVALID_DATE)) {
              return "INVALID_DATE";
          }
          return String.valueOf(getValue().getTime());
      }
docs: Returns the value of the date box as String in form of a long.<p>
  
  @see org.opencms.gwt.client.ui.input.I_CmsFormWidget#getFormValueAsString()