L'ho editato in questo modo:

@echo off
:: variables
set file=c:\a
set ext_file=txt
SET dateNtime="%date:~6,6%-%date:~3,2%-%date:~0,2%_%time:~0,2%-%time:~3,2%"
set backupcmd=ren
echo ### Backing up directory...
%backupcmd% "%file%.%ext_file%" "%file%_%dateNtime%.%ext_file%"

echo Backup Completato!
@pause

ma mi dice Sintassi del comando errata o simili...