It seems that by executing code in PS0 and PS1 variables (which are eval'ed before and after a prompt command is run, as I understand) it should be possible to record time of each running command and display it in the prompt. Something like that:

user@machine ~/tmp
$ sleep 1

user@machine ~/tmp 1.01s
$

However, I quickly got stuck with recording time in PS0, since something like this doesn't work:

PS0='$(START=$(date +%s.%N))'

As I understand, START assignment happens in a sub-shell, so it is not visible in the outer shell. How would you approach this?