-type aws_datetime() :: string(). % "YYYYMMDDTHHMMSSZ"
-type iso_datetime() :: string(). % "YYYY-MM-DDTHH:MM:SSZ"

-record(credentials,
        {expiration :: undefined | iso_datetime(),
         security_token :: undefined | string(), % required when using temporary credentials
         secret_access_key :: string(),
         access_key_id :: string()}).