Even if it has nothing currently in there?
So if a address was owned by me at that time but it has nothing in there currently, I will still own CLAMs? Does it have to have some bitcoins in it at that date, or just the ownership of the address?
Sorry for some silly questions.
Yes, also if it has nothing currently in it (and you should empty it by sending to a different address before entering the private key to claim CLAMs). But only if it had some amount "non-dust" in it at the date above, and you have the private key to it now.