Adrienne is always two months away from her birthday the moment you get her, so how do npcs birthdays work? Would, for an example, Kymanto slaves always keep their age until player buys them or is it tied to the in-game date?
NPCs are x number of years and y number of days old when they are created.
Most of the unique npcs have those numbers hard coded (as opposed to randomized).
As result, they will always be the same amount of days away from next birthday, regardless of which date they were created (which typically happens when they are first encountered or added to the household).
The most obvious example would be Lovisa who is always bought on her 18th birthday, regardless of when mc decides to buy her.
Edit: I think that I will look into the possibility to add birthday randomization for newly added npcs in the automation mod. It should in theory be very simple to do.