[exim] DKIM timestamp and expiration tags

Hi folks,

DKIM signing and verification seems to work fine on my Debian wheezy
systems running Exim 4.80. However, a friend of mine pointed out that
the DKIM header in my outgoing messages does not contain tags for a
signature timestamp (t) and a signature expiration (x). While these
tags are recommended by RFC4871, Exim does not appear to include
support for them. Can anyone confirm this? If true, why are they are
not supported?

Thanks,

Jaap