The spectator is handed an open brass combination padlock. They are then asked to link their ring onto the lock and snap it closed. The combination is mixed up and the spectator is asked to try and guess what it is. Unfortunately, they don't have much luck. The performer comes up with an idea and asks the spectator to put their year of birth into the lock. This enables the lock to open, so the spectator gets their ring back.
This specially gimmicked padlock does all the work for you. You don't need to know the spectator's year of birth. In fact, any date will work. The number doesn't have to be a year of birth, it could be the first four digits of a phone number, an important year to a spectator (more personal!) ... anything you want!
The incredibly clever method to this trick leaves it open to a range of possibilities for both magic and mentalism.