Differential backups with rsync

I already knew how to backup a directory with rsync:

$ rsync -Pav --delete /path/to/dir1/ /path/to/dir2/

This is nice and good, but what if a file in dir1 becomes corrupted? Its corruption will be copied verbatim to dir2 and the backup will be of no use. Fortunately, there is a way to do differential backups with rsync:

$ rsync -Pav --delete --backup \
  --backup-dir=/path/to/versions_dir \
  --suffix='_$(date +"%Y%m%d%H%M%S")' \
  /path/to/dir1/ /path/to/dir2/