Time is money. But how much? What is money in the future worth to you today? This question of "present value" arises in myriad economic activities, from valuing financial securities to real estate transactions to governmental cost-benefit analysis—even the economics of climate change. In modern capitalist practice, one calculation offers the only "rational" way to answer: compound-interest discounting. In the early modern period, though, economic actors used at least two alternative calculating technologies for thinking about present value, including a vernacular technique called years purchase and discounting by simple interest. All of these calculations had different strengths and affordances, and none was unquestionably better or more "rational" than the others at the time. The history of technology offers distinct resources for understanding such technological competitions, and thus for understanding the emergence of modern economic temporality.