def validate_datetime(value)
      DateTime.rfc3339(value)
    rescue ArgumentError => e
      raise ValidationError, e.message
    end
def validate_datetime(value)
      DateTime.rfc3339(value)
    rescue ArgumentError => e
      raise ValidationError, e.message
    end
Validate date and time string format following RFC 3399
 (see https://tools.ietf.org/html/rfc3339)