The batteries are hot swappable, so as long as batteries are changed when the display indicates low batteries (every 20-40 years), TSL won't lose count.
In the unlikely event that the batteries and the backup capacitors completely drain out, your TSL will lose count and will need to be serviced.
We can revive it with a special device to reprogram the chip to the current time. It then reads the launch timestamp that was burned into the chip when the pin was pulled and displays the appropriate time again.
We will have several of these devices scattered around the world in trustworthy hands, so the longevity of your TSL isn't dependent on just us.