Time shapes almost everything we do, yet the way we measure it is not as natural as it seems. Throughout history, different peoples have tracked their days using the sun, the moon, or a combination of both. Some divided the year into months of equal length, while others did not, and even the number of days in a week has varied from one culture to another. Nature provided the signs, but people chose how to count it.

According to Elizabeth Cohen, a political science professor at Boston University, every way of measuring time is arbitrary. She argues that no unit is truly natural because people could have chosen to divide time in many other ways. For most of human history, timekeeping simply followed nature, such as the phases of the moon and, in farming communities, the seasons for planting and harvesting.

This changed with industrialization. Factories needed workers to arrive for fixed shifts, so people began paying close attention to the clock rather than to natural signs. Michael O'Malley, a history professor at George Mason University, notes that industrial society taught people to experience time as something regular and unvarying.

Economic and political decisions also shaped time on a larger scale. In the United States during the 1800s, cities kept more than 40 different local times, which made train travel confusing. To fix this, American railroad companies agreed on a four-zone system in 1883. In 1884, an international conference chose Greenwich, England, as the basis for global timekeeping.