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To have an accurate and unambiguous definition of time is a matter not only for scientists and experts. A universally recognized and very accurate reference time is in fact at the base of most infrastructures of our society. All cellular and wireless networks, for example, are based on careful synchronization of their nodes and base stations (obtained receiving GNSS signals, as we will see). The same is true for electric power distribution networks. Surprisingly, even financial transactions and banking and stock markets all depend on an accurate time reference, given the extreme volatility in equity and currency markets, whose quotations might vary within a few microseconds. The history of the measurement of time is as old as the history of human civilization. But the major impetus for the development of ever more accurate techniques for measuring time came from the need to determine one's position (particularly longitude) aboard a ship in the open sea. In 1761, a self-educated Yorkshire carpenter and amateur clock-maker named John Harrison built a special mechanical clock to be loaded on board ships, called the “marine chronometer”, capable of losing or gaining no more than one second per day (an incredible accuracy for that time). From then on, time and positioning became irreversibly connected.