Ian Murdoch is indeed a loss. Even though he has not been involved directly with Debian for some time, he made great contributions to Linux and FOSS, or which Debian may be his longest-lasting. Nevertheless, I'm not sure you can do what you want to do, which, as I understand it, is to sort of "freeze-frame" a specific iteration of Debian.
Debian doesn't quite work like that. Debian Stable is Debian stable and Debian testing is a moving target. Finding a version within a specific narrow timestamp would be quite challenging.
You can install the latest Stable
, then update it to testing
, but I suspect that you cannot freeze it in time.
Good luck and welcome to LQ.