although you might not notice, it occurs frequently all around you, most commonly in your laptop or mobile phone. Other clocks, especially those used for public purposes (for example, railway clocks), are synchronized by a radio signal. Accuracy comes at a cost. A highly accurate oscillator (such as an atomic clock) can be set once and it will maintain very accurate time for an extended period; however, this type of oscillator is very expensive to buy and maintain. A far more cost-effective solution is to put cheaper "good enough" oscillators in our devices and use some form of time synchronization to periodically reset them to the correct time. This compromise will be revisited later, as it is an important factor in building a synchronization network: inherently better clocks versus better correction mechanisms. Now it should be apparent that to align time accurately, you need a combination of all forms of chronization: frequency, phase, and time. The oscillators must spin at the correct frequency, the clocks must tick over at the correct moment, and the date on the watch face must correctly display what everybody accepts to be the correct time. Which raises the next question. What exactly is the time everybody agrees on? 1.3 What Is Time? The basic unit of time that most directly affects us all is the day, which is controlled by the rotation of the earth and its orbit around the sun. Even with something seemingly immutable as the day, there is nothing absolute about time. Rather, it is an agreement among us all about what time is just as everybody accepts what the meter or kilogram is. This agreement covers how we define both larger periods of time (the calendar) and smaller periods of time (such as the second). Originally, metrologists, the people who develop standards for our units of measure, defined the second (before 1956) to be 1/86,400 the value of a mean solar day, and developed what is known as universal time (UT). The main problem with tying a definition of the second to the earth's rota- tion is that