In these two lectures Peter Diamond explores how time is modeled in theoretical analyses of individual industries and of an entire economy. In the first lecture he considers equilibrium in a single market by examining the distinction between the short run and the long run in Marshallian analysis. He proposes an explicit modeling of time in place of Marshall's use of different atemporal models for different time frames. A model with different expansion paths for different firms and models of price competition with incomplete information are presented. Data on job creation and destruction and data on price changes are examined. In the second lecture he turns to models of an entire economy, and begins by considering how and why models of an entire economy should differ from models of a single industry. Both cyclical and seasonal data on the behavior of macro-economics are examined. The Arrow-Debreu and Hicksian ISLM models are compared with explicit-time models of the command over purchasing power. Professor Diamond ends by indicating a direction for future research that might yield a more integrated economics.