How do we measure time? As the Millennium approaches great significance will be placed on the symbolism of midnight on 31 December Greenwich Mean Time. But what is the history of time and how do we measure it? This Gem, building on the exhibition at the Greenwich Observatory, tells the story of time, tracking the variety of different ways that have been used to measure time from Egyptian water clocks and speculation about the significance of prehistoric sites through the calendar changes to the international standard definition of time based upon the vibration of a caesium atom. Heavily illustrated both with diagrams and artifacts, the history of time is brought alive. It is the first of a continuing series of Gems on issues in popular science.