Run the below shell script from root's crontab:

  #!/bin/sh
  DAYS=7
  DIRS="/tmp /var/tmp"
  find $DIRS -depth -fsonly vxfs -fsonly hfs -atime +$DAYS \
  -exec rm -rf {} \;

The -depth option ensures no directory is removed before its contents,
-fsonly options are to avoid attempting        ttttttttto remove files
beneath NFS or
CDFS mountpoints that might exist.