List branches whose head was committed prior to a given date

git-old-branches.sh

#!/bin/bash
function git-old-heads {
    git show-ref --heads | grep -E "$(git rev-list --all --before="$1" | cut -d' ' -f2 | xargs | sed -e 's/ /|/g')"
}

function filter-branch-names {
    cut -f2 -d' ' | sed -e 's|refs/heads/||g'
}

git-old-heads "$1" | filter-branch-names

# eg. git-old-branchs "2 months ago"