code: @Override
      public ValidationResult validate(Object value) {
          if (value instanceof org.joda.time.DateTime && value.toString().endsWith("Z")) {
              return new ValidationResult.ValidationPassed();
          } else {
              return new ValidationResult.ValidationFailed(value + " is not a valid date!");
          }
      }
docs: Validates: Object is not null, of type org.joda.time.DateTime and
  the String representation is in UTC.
  
  @param value The object to check
  @return validation result