The concept of time has been central in economics since at least Alfred Marshall. The distinction between the short-run and the long-run is rather well-accepted and uniform in microeconomics and depends on the ability of a firm to adjust or vary its factors of production. The short-run is nearly always defined in microeconomics as that time period in which at least one factor of production is fixed while the long-run is conceived as a time horizon sufficient such that all factors of production may be changed. A survey of some principles and intermediate-level textbooks suggests that no such standard definitions of time horizons exist within macroeconomics, and the corresponding treatments of aggregate supply are also quite diverse. Perhaps the most common distinction between time periods is based on price flexibility. Indeed, Gwartney, Stroup, Sobel and Macpherson [Economics: Private and Public Choice, 2006], Hall and Papell [Macroeconomics: Economic Growth, Fluctuations, and Policy, 2005], Mankiw [Macroeconomics, 2003], McConnell and Brue [Economics: Principles, Problems, and Policies, 2005], O’Sullivan and Sheffrin [Economics: Principles and Tools, 2006], and Samuelson and Nordhaus [Economics, 2005] all define the short-run as that time period in which some prices are fixed while the long-run is specified as a time period sufficient enough that all prices are variable. Blanchard [Macroeconomics, 2006], Gordon [Macroeconomics, 2003], and Williamson [Macroeconomics, 2005], however, distinguish between the short-run and long-run based on calendar time, and Froyen [Macroeconomics: Theories and Policies, 2005] provides alternative conceptions of the two time horizons. With respect to aggregate supply, there appears to be universal agreement that the long-run aggregate supply curve is vertical at the full-employment level of real gross domestic product. On the other hand, while Hall and Papell depict the short-run aggregate supply curve only or always as horizontal, Blanchard, Gwartney et al., McConnell and Brue, O’Sullivan and Sheffrin, Samuelson and Nordhaus, and Williamson show it as positively sloped. Lastly, Froyen, Gordon, and Mankiw present the shortrun aggregate supply curve as both horizontal and positively sloped. The purpose of this note is to suggest an approach which may provide some standardization of time periods for macroeconomics. More specifically, we propose and define four distinct time periods for macroeconomics: The extreme short-run, the shortrun, the long-run, and the extreme long-run. The behavior of aggregate supply is linked to the different time horizons. To begin, we assume or consider five parameters on the economy’s supply side: The quantity of labor, the quantity of capital, the quality of these factors of production, or technology, the prices of these factors, and commodity prices or the general price level. The different time periods may then be treated as a progressive and systematic relaxing of restrictive assumptions with respect to these parameters. For example, we define the Atlantic Economic Journal (2006)34:235Y236 * IAES 2006 DOI: 10.1007/s11293-006-9010-5