#!/bin/zsh
t=$(< /usr/portage/metadata/timestamp)
(( age=$(date +%s) - $(date --date="$t" +%s) ))
(( d=age/86400 )) ; (( age%=86400 )) ; (( h=age/3600 )) 
(( age%=3600 )) ; (( m=age/60 )) ; (( age%=60 ))
printf "tree from %s aged %id %02ih %02im %02is\n" \
	"$(date --date="$t")" d h m age