July 16, 2025 11:50 PM (GMT+2) For now, I have no idea what date could be related to this safe.

August 16, 2025 5:51 PM (GMT+2) After knowing that I could infer the code from elements in the room (see How to unlock safes), I thought to try transforming the square where the king is in the Chessboard at Study (D8) to a date. The only sensible way to do it is “D(ecember) 8”. I tried 1208 and to my surprise, it worked! Mystery solved.