I have the following rsync command:
0 0 * * * rsync -av --delete /media/VIDEOS/ /media/lacie1/backup/videos/
I want to set it so instead of doing a replication like what it's doing now, it does a normal backup and only deletes file from the destination only after they haven't been found on the source for X days (like 7 days). This is to give a larger window of recovery if files go missing, but without keeping loads of duplicate files as the files being backed up get moved around a lot.
I wouldn't reinvent the wheel; for example rdiff-backup is rsync based and can do what you need, among many other things.