#/bin/bash

[[ -n $1 ]] && goal="$(date +$1)" || goal="tomorrow 00:00"
LANG=C date -d "$goal" >/dev/null || exit 1

function countdown_negative() {
  echo "$(basename $0): invalid countdown '${remainingsec}'" >&2
  exit 2
}

remainingsec=$(($(date -d "$goal" +%s) - $(date +%s)))
[ $remainingsec -lt 0 ] && countdown_negative

H=$(($remainingsec / 3600))
remH=$(($H * 3600))
[ $H -lt 10 ] && H="0$H"

m=$(($(($remainingsec - $remH)) / 60))
remm=$(($m * 60))
[ $m -lt 10 ] && m="0$m"

s=$(($remainingsec - $remH - $remm))
[ $s -lt 10 ] && s="0$s"

echo "$H:$m:$s"