The Doomsday Clock is a symbolic object representing the (approximate) time since the last global catastrophe. The measurement is rendered in "Minutes After Midnight"—the further the clock is from midnight, the longer it has been since the last cataclysm. Originally (pre-Excession), the clock was used to measure the estimated proximity of a global nuclear war or climatological catastrophe. It has since been expanded to cover major spatio-temporal disruptions, significant geological upheavals, and various levels of extinction event.
Currently, the most widely-accepted Doomsday Clock is published by the Xenotemporal Institute of Nairobi. This is based, in part, on its proximity to the equator (and therefore distance from any of the polar events). Current reckoning puts us at four minutes after midnight.