Add the following snippet at the bottom of ~/.zshrc
file.
RPROMPT='[%D{%L:%M:%S}] '$RPROMPT
RPROMPT="[%D{%f/%m/%y} | %D{%L:%M:%S}]"
When you are done, run $ source ~/.zshrc
to see the effect. If it doesn't work, quit iTerm2 and open it again.
Just FYI, this doesn't have anything to do with iTerm2. It has its own timestamp feature that works independently of the shell prompt.