Bash functions

a set of commands that can be called numerous times.
https://linuxize.com/post/bash-functions/

add the code lines to your ~/.bashrc 

countdown() {
    start="$(( $(date '+%s') + $1))"
    while [ $start -ge $(date +%s) ]; do
        time="$(( $start - $(date +%s) ))"
        printf '%s\r' "$(date -u -d "@$time" +%H:%M:%S)"
        sleep 0.1
    done
}

stopwatch() {
    start=$(date +%s)
    while true; do
        time="$(( $(date +%s) - $start))"
        printf '%s\r' "$(date -u -d "@$time" +%H:%M:%S)"
        sleep 0.1
    done
}

# sleep 0.1` will make the system wait for 1/10th second between each run so you don't spam CPU)