Backing up by Date - can anyone please assist?

I work at a University and my department keeps lots of video data that is added to v frequently. We save all of the videos onto a 4TB WD MyBook external drive. The video data is sensitive and confidential and so the MyBook is kept locked away in a cupboard each evening. Therefore we save to the video files to the MyBook manually as we go along rather than having them perform an automatic backup overnight.
The video files are saved into different folders - 1 folder for each person being filmed. There are about 70 folders.
As a backup to the MyBook we’ve bought a second 4TB MyBook. I’ve successfully backed up the original MyBook onto the second MyBook - a total of 2.5TB of data.
I’ve been asked to perform this backup once a month however it seems ludicrous to copy over the whole drive again when only the past month’s data is all I really need to copy over. ie if I was performing the backup today (Jan 8th) then I only want to copy over the files from Dec 8th - Jan 8th.
I’d be extremely grateful if anyone could explain in layman’s terms how I can achieve this. I’ve looked into the software and can’t see any pointers at all.


I have not tried this, lets see if another user can share some information on this matter.