Time concepts are named differently across the world's languages. In English, the names for days of the week and months of the year are opaque—to people learning and using English, there's no obvious reason why Friday or September have the names they do. But in other languages, like Chinese, time concepts have numerically transparent names—the days of the week and months of the year are named using sequential numbers. We investigated whether having opaque versus mathematically transparent time concepts affects how people reason about time. Results show that Chinese speakers are more likely to spontaneously employ arithmetic when doing temporal calculations, which in turn improves the speed and accuracy of some time calculations. English speakers appear to use other strategies, such as sequential recitation.