Yeah there are a few puzzles in the game that could have a second pass. This was just a school project, so we never got around to making a second build. Basically, you can see that the wife's birthday is coming up on the 13th of April. So you just need to determine what year she was born. Through another conversation and by looking at the calendar, you learn that she was born in 1901. So the answer to the safe would be 4-13-1
I agree it's a bit confusing, and I'll be adding a walkthrough of the game for anyone confused about any of the puzzles.