How to Change File Modified Date on All Files to Midnight

Bit of an obscure one, but I searched Google without finding a specific example.

After back up, I want to set all my files in a given directory to be modified at midnight of the current day. The real modified date is not very important to me, so this command suffices. I have a specific reason for this process, though it’s quite unusual.

Anyway, here goes:

As ever, use with caution / practice on a copy of your data first.