Zero Day was the designation given to the day when, according to projections, life on Earth would be eradicated by the Faro Plague. In actuality, some life persisted beyond Zero Day, as a small number of humans lived out the remainder of their natural lives in hermetically sealed habitats, safe from the hostile robots and uninhabitable environment.
In quarter 4 of 2064, a unit of Chariot robots designated the Hartz-Timor Swarm stopped responding to commands and became an independent entity, answering only to itself. Utilizing their ability to replicate and convert organic matter to fuel, and protected from hacking by the virtually unbreakable security built into their OS, the robots quickly multiplied and overran the planet, consuming the biosphere until it was completely destroyed, exterminating all life and leaving the planet sterile.
The designation Zero Day was used by General Aaron Herres, Chairman of the Joint Chiefs of Staff of the United States Armed Forces, in his presentation to the prospective candidates for Zero Dawn, an above top-secret plan to create an automated terraforming system that would first brute-force the Faro Plague's deactivation codes, then broadcast them to shut the swarm down, and then finally restore life to the planet. Candidates who agreed to participate in the project were slated to live out the remainder of their natural lives in a hermetically sealed habitat designated Elysium. However, unforeseen events caused the heads of each section of the project, designated as Alphas, to be sealed in the facility where the system was based, as they had to keep working on the system beyond the time when their families and loved ones were moved into Elysium. Thus, these and other humans in other habitats lived beyond Zero Day, as the last living organisms on the planet. Their deaths marked the de facto end of life on Earth.