I just stuck this in my PS1:

    hr() {
        printf '=%.0s' $(seq $((${COLUMNS} - 20)))
    }

    PS1="${GREY}\d ${RED}\$(hr)${GREY} \t"

It dynamically adjusts to the width of the terminal, and looks like this:

    Mon Feb 10 ======================================== 16:30:30

My complete BASH prompt: https://bitbucket.org/wyatt/dotfiles/src/25cb260a05b68dd81a5...