I think you need to detail a bit more about what you’re doing, I don’t think I understand it.
Unmanaged Assets are those devices without a BES Client - they won’t evaluate relevance and won’t become members of a group.
The “OS Install Date” property is read from the Registry - is it giving you the wrong value? I could see that being the case depending on how you are deploying or cloning your OS.
To detect “new to BigFix” clients, we usually use the BigFix site subscription times - like
(Now - minimum of subscribe times of sites) < 10 * day
Edit: ‘subscribe times’ not ‘subscription times’