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))
(( $D > 0 )) && printf ‘%d days ‘ $D
(( $H > 0 )) && printf ‘%d hours ‘ $H
(( $M > 0 )) && printf ‘%d minutes ‘ $M
(( $S > 0 )) && { (( $D > 0 || $H > 0 || $M > 0 )) && printf ‘and ‘
printf ‘%d seconds\n’ $S; } || printf ‘\n’
}

-bash-4.1$ displaytime 604800
7 days
-bash-4.1$ displaytime 604801
7 days and 1 seconds
-bash-4.1$ displaytime 800
13 minutes and 20 seconds
-bash-4.1$ displaytime 780
13 minutes