Backup Changed Files To 3rd Directory?

I'm trying to set up a backup system where I copy changed files to a zip file or directory outside of the regular backup set.In other words,And that would replace anything in D that had been changed in C. Great.But what if I want to save those changed files in C to echanged/%date%? I can't see a way to do this with Robocopy.And in case it isn't obvious, the advantage of using this method is you get redundant backups using much less space. In my experience backups are used more often to recover corrupted files than corrupted harddrives. And the problem with simply mirroring a drive is if the backup runs after the corruption, your backup is now corrupted too. And I know I could keep multiple sets of the whole backup, but that takes mucho space.