#!/bin/bash

function displaytime {
  local T=$1
  local D=$((T/60/60/24))
  local H=$((T/60/60%24))
  local M=$((T/60%60))
  local S=$((T%60))
  printf '\r%s[' $PREFIX
  (( $D > 0 )) && printf '%dd ' $D
  (( $T > 3599 )) && printf '%02d:' $H
  printf '%02d:%02d]%s\r' $M $S $POSTFIX
}

while true; do
  displaytime "$SECONDS" >&2
  sleep 1
done