The U.S. Department of Homeland Security has issued a memorandum for owners and operators of U.S. critical infrastructure that use Coordinated Universal Time (UTC) from Global Positioning System (GPS) devices. The memorandum is intended to assist owners and operators with preparations for a GPS “Week Number Rollover” that is scheduled to occur on April 6, 2019, as this event may impact the reliability of the reported UTC. DHS strongly encourages owners and operators to act on the recommendations it provides in this document, which begin with investigating and understanding possible dependencies on GPS for obtaining UTC.