code: @Override
      public void validate() throws Exception {
          super.validate();
          new SimpleDateFormat(this.getInput().getValue());
          Assert.assertFalse("Unique attribute isn't applicable for fixed value",
                  this.getInput().isUnique());
      }
docs: /* (non-Javadoc)
  @see com.github.mkolisnyk.aerial.expressions.ValueExpression#validate()