Backup files only that are newer than a specific date?

I have made a backup B1 on my external hard drive on date D. This hard drive is now in a different location (not at my house). Now I want to make a backup B2 on another external drive, but only of those files that where changed after the date D. As soon as I have B1 back in my hands, I will update B1 and delete B2. The point of B2 is to have security regarding the changes since date D, until I can update my backup properly again. How can I do this even if I do not have B1 at hand?