code: public void setFormValueAsString(String value) {
  
          if (!CmsStringUtil.isEmpty(value)) {
              try {
                  long time = Long.parseLong(value);
                  setValue(new Date(time));
              } catch (NumberFormatException e) {
                  // if the String value is none long number make the field empty
                  setValue(null);
              }
          } else {
              // if the value is <code>null</code> make the field empty
              setValue(null);
          }
      }
docs: Expects the value as String in form of a long.<p>
  
  @see org.opencms.gwt.client.ui.input.I_CmsFormWidget#setFormValueAsString(java.lang.String)