Time gives rhythm to the world around us. But how do we really know what time it is? Professor Brian Cox goes in search of the answer to a question you wouldn't give a passing thought. From our historical dictator of time - the Sun - to the world's irregular flutters and the physical definition of a second by an atomic clock, Brian discovers telling the time is a tricky business. But why do we need to know what time it is so very accurately? Without correct time signals, aircraft, telecommunications systems, the world's global financial systems, global positioning and many other systems we use and rely on everyday wouldn't work. They all depend on knowing the time. Exactly. In this entertaining film, Brian pushes our understanding of time to the depths of our imagination.