Automator rename finder item date and time
Check the answer found here

https://apple.stackexchange.com/questions/52535/clean-up-file-names-using-automator

I changed the bash script code to include the date code.

shopt -s extglob
dcode=`date '+%m%y'`
for f in "$@"; do
   f1="${f%.*}"
   f2="${f##*.}"
   if [ "$f2" == "$f1" ] ; then     
       echo "$f" "${f1}_${dcode}"
   else
       echo "$f" "${f1}_${dcode}.${f2}"
   fi
done


The code will should work even with files that do not have extensions. 

Replace the commands echo with mv to actually get the code to do anything.

Hope that helped.