[groff] Changing A Defined String

[Robert Thorsby]

Temperatures here in AU have been in the range 35-45 for the past week  
and are forecast to be the same for the next week.  So with no  
inclination towards serious work I have turned my attention to  
rewriting my letterhead shell script, as one does.

My letters carry the date in full -- Thursday, 25 January 2018 -- which  
is placed into the preamble of the groff file via my shell script:

date --date="..." "+.ds DATE*FULL %A, %-d %B %Y" >> draftletter.gr

But occasionally I also want to use the date in a ``shorter'' version  
that depends on the ``full'' version.  I appreciate that I can simply  
create two definitions, but once defined they would be independent of  
each other.

.ds DATE*FULL Thursday, 25 January 2018\"  from the shell script
.ds DATE*SHORT 25 January 2018\"           from the shell script
[each can be subsequently edited without reference to the other]

What I'm after is something like:
.ds DATE*FULL Thursday, 25 January 2018\"  from the shell script
.ds DATE*SHORT \*[DATE*FULL]\"			static entry in groff  
file
DATE*SHORT="$(sed 's/.*, //' <<< "$DATE*FULL")"	static entry in groff  
file

Can anyone assist with proper groff request(s) to replace the bashism  
so that any amendment to DATE*FULL is carried through automagically to  
DATE*SHORT?  I don't use any macro package.

TIA,
Robert