Dan Smith suggested as a way to get the proper syntax for a Resent-Date:
S> DATE=`date '+%a, %d %h %Y %H:%M:%S %Z'`
Thanks, Dan, but I'd still have to double-check the man page for date(1)
locally to be sure the macros are the same. (I think most North American
implementations have %T as shorthand for %H:%M:%S.) If someone comes up
with a way to do it within formail without another fork, I'm all ears.
Stephen van den Berg answered my original question this way,
B> Simply provide one of the Resent- fields on the outgoing mail, sendmail
B> will add the Resent-Date: field all on its own.
It's SUPPOSED to, and I expected it to, and you count on it for SmartList,
but as things turn out, it doesn't happen that way here. WWA runs Smail
184.108.40.206#29.2, and it adds Resent-Message-Id: but not Resent-Date:. That's
why I want a way to put it in myself.
It appears that I'll have to run date. Darn.