code: func ValidateRFC3339TimeString(v interface{}, k string) (ws []string, errors []error) {
  	if _, err := time.Parse(time.RFC3339, v.(string)); err != nil {
  		errors = append(errors, fmt.Errorf("%q: invalid RFC3339 timestamp", k))
  	}
  	return
  }
docs: // ValidateRFC3339TimeString is a ValidateFunc that ensures a string parses
  // as time.RFC3339 format