That's the worst solution for backups I have ever seen. You have no way
to verify the data you are storing. You have no way to restore data if
the backup was foobar. Why not use your head instead of your ass to
think with before recommending something like a cron job and rsync for
a backup solution.

man tar

Look for --newer (-N)

Store backup archives somewhere safe, not on a hard drive.

That's not even the best solution, but it's a lot better then hoping
rsync will somehow magically know that the data it is writing is "good".