Hey August!

 AA> OpenXP does a fine job of it.

I'd like to see the routine.  Best I can think of is to combine the MSG header
datetime stamp with the TZUTC and then use that resulting string to make the
adjustment.  Your combined msg header date and TZUTC string would be "13 Nov 20
 18:58:54+0100" which requires extra steps to assemble as well as to tack on
the '+' character onto the TZUTC which is lacking and NEEDED to do proper
conversions.  A rfc-3339 datetime stamp is vastly superior and contains
everything needed.  In your case it would be "2020-11-13 18:58:54+0100" which
is the same as "2020-11-13 09:58:54-08:00" relative to 1:153/7001.2989.

Anyhow it is doable but requires additional processing in order to honour an
obsolete datetime stamp (two digit year) and a corrupted utc offset which lacks
the required '+' prefix for timezones east of prime meridian.

Life is good,
Maurice

... Freond deah feor ge neah; byð near nyttra.
    A friend is useful, far or near; the nearer the better.
--- GNU bash, version 5.0.18(1)-release (x86_64-motorshed-linux-gnu)