Hallo mark!

 ml> export TZUTC=$(date +"%z" | sed 's/\+//')

Yes but then $TZUTC won't work for normal dating applications as shown by using
 my FTN date and what would be a corrupted TZUTC since all my systems are set
to UTC;

-={ '<Esc>:read !date --date="18 May 19  15:58:01 0000"' starts }=-
date: invalid date ΓÇÿ18 May 19  15:58:01 0000ΓÇÖ
-={ '<Esc>:read !date --date="18 May 19  15:58:01 0000"' ends }=-

Without the '+' character (when applicable) produces further error unless
compensated for.  Personally I don't trust date/time and tend to just ignore it
 for the mostpart, especially 2 digit year based ones.  It is simpler - and I
believe better - to completely ignore it rather than create a worse bug at the
expense of exporting obviously corrupted data.  Also, and more importantly, the
 TZUTC is part of the msg_body and thus by my recongning shouldn't be tinkered
with even it is obviously corrupted such in the case of UTC.  By simply not
adding one is the correct answer methinks.  However I'd be receptive to
something along this line instead;

  @RFC-3339: 2019-05-18 15:58:01 +0000

which shows a proper full date without the TZUTC corruption or the English
abbreviations which cause even more grief which I won't get into here.  The
above kludge can be created using the strftime specifiers +"%F %T %z".  So for
coreutils 'date' it could look like this;

  export RFC3339=$(date --date="@${UNIXTIME}" +"%F %T %z"

Note in the above that $unixtime is the time in seconds when this reply was set
 in motion = 1558195081 and is used to set the FTN datetime stamp as well. 
That might be a more useful kludge and could also be set to use nanoseconds for
 greater uniqueness as follows;

  export UNIXTIME=$(date +"%s.%N"

My vote would be to supply the unixtime (with or without nanoseconds) and then
use that to produce whatever datetime stamp the end user wishes to display the
date and time at their end without having to alter the original packed
message(s).

Het leven is goed,
Maurice

... Huil niet om mij, ik heb vi.
--- GNU bash, version 5.0.7(1)-release (x86_64-pc-linux-gnu)
 * Origin: Little Mikey's EuroPoint - Ladysmith BC, Canada (2:280/464.113)