[_kst_]

I have a better (at least cleaner IMHO) workaround:

    PROMPT_COMMAND='printf -v today "%(%F)T" -1'

printf's "-v" option was added in bash 4.0.

printf's "%(...)T" format was added in bash 4.2.

The "-1" argument became optional in bash 4.3.

So here's what I now have in my .bashrc :

    if [[ "$BASH_VERSION" > "4.2" ]] ; then
        PROMPT_COMMAND='printf -v today "%(%F)T" -1'
    else
        PROMPT_COMMAND='today=$(date +%F)'
    fi

(The "> 4.2" test is true for bash 4.2 and later, since the value of $BASH_VERSION for 4.2 is "4.2.0(1)-release". The ">" does a string comparison. I'll have to revisit this when and if there's a bash version 10.0, probably using $BASH_VERSINFO.)