I think you want to do:
export PROMPT='[%n]%t '
PROMPT='[%n]%t ' setenv doesn't seem to work with Zsh version 5.0is a csh command.8 The equivalent in Bourne-like shell would be export, which is what I tried it withbut here you don't want to export that variable to the environment, just define the $PROMPT shell variable (typically in your ~/.zshrc).
Try man zshmisc and look for the section "SIMPLE PROMPT ESCAPES" (or run info zsh 'prompt expansion') to see all of the escape-sequences available. Also, take a look at zsh's RPROMPT environment variableRPROMPT variable. It can be useful and fun.