Given the sometimes impressive fraction of duds among bombs, I am not sure I trust a mine that supposedly deactivated itself on a timer. It may work 9 times out of ten. But it doesn't really make a big difference if it's the first, second or the tenth mine that gets you and rips off your leg.
They deactivate by discharging their power supply, after which they are lacking the stored electrical energy needed to trigger the electrical detonator.
It's not fool-proof, but a lot better than chemical/mechanical mechanisms that try to go inert.
