@Vagabond this fields are rare-used, so I just was lazy to finally implement `datetime_to_bin_timestamp/1` for `t` and `x` tags. It should return UNIX timestamp for given datetime.
`z` field is very rare used and require fair `dkim-quoted-printable` implementation first (see above).
If you think this should be implemented, I will do, but `t` and `x` are easy to implement and `z` is hard.