International Telemetering Conference Proceedings / October 25-28, 1999 / Riviera Hotel and Convention Center, Las Vegas, Nevada / This paper describes the current internationally recognized atomic time scales of International Atomic Time (TAI), Coordinated Universal Time (UTC), and Global Positioning System (GPS) Time as well as solar based Universal Time. The concept of Leap Seconds and the differences between the time scales are discussed. A brief history of the international agreements that created organizations responsible for maintaining these time scales is provided. A brief review of the GPS 1999 (Week Number Roll Over) WNRO with its potential GPS user problems is provided. Prudent personal precautions are proposed for the Year 2000 (Y2K) Rollover.