Above the entrance to the Historical Green Vault is a clock. Its purpose: one can see the actual time in hours and minutes and compare it to their own time slot. What the clock is missing, however, is a seconds hand. Artist Olaf Nicolai has designed an identical timepiece for the opposite exit that only has a seconds hand. The seconds shown correspond perfectly to all time zones. To be sure of the current time when exiting, it helps to look back to the point where the visit began.