Time in evolution is arithmetic, with units of equal measure added repeatedly. Time is directional in the sense that units are added but never subtracted. The time scale of a generating process may be different from the time scale of an observation, and the distinction is important. Natural selection and random change are generation-scale processes acting on the parade of phenotypes produced in each generation. Generation times of organisms vary in length from minutes to decades. Close correlation with body size in groups like mammals enables unknown generation times to be estimated from size. Differences in organisms over spans of ecological and evolutionary time represent an accumulation of changes happening on a time scale of generations. Microevolution and macroevolution parallel ecological and evolutionary time, but these terms are also used to distinguish evolution within species from evolution between species and higher taxa. Evolutionary change is often imagined and modeled on a ‘game board’ or simulation space of form and time that are equivalent in length and width, but these may differ greatly.