In November 1897, in a field near the village of Coligny in eastern France, a local inhabitant unearthed two strange objects. One was an imposing statue of Mars, the Roman god of war. The other was an ancient bronze tablet, 5 feet wide and 3.5 feet high. It bore numerals in Roman but the words were in Gaulish, the extinct version of Celtic spoken by the inhabitants of France before the Roman conquest in the first century B.C. The tablet, now known as the Coligny calendar, turned out to record the Celtic system of measuring time, as well as being...