People make sense of the world by comparing and relating new information to their existing landmarks. Each individual may have different landmarks, developed through idiosyncratic experiences. Identifying specific events that constitute landmarks for a group of learners may help instructors in gauging students' prior knowledge and in planning instruction that helps students build additional landmarks events. This paper proposes an operationalized definition for collective landmarks based on importance, accuracy, and precision. Including precision in the definition allows landmarks to be characterized for a group rather than an individual. This study evaluated the ability of undergraduate students in an interdisciplinary course to estimate scales of time related to major cosmological, geological, and historical events. Individual students responded to replicate questions in different formats with the same answers, indicating the testing format was valid. The students' estimates were then used to determine collective landmarks. The number of collective landmarks increased between the pretest and posttest. Collective landmarks included extremely ancient events (Big Bang, formation of the solar system) or relatively recent ones (Cold War, the age of empires, emergence of nation states). Intermediate events had low accuracy, low precision, or both. These data indicate that lecture courses can teach students collective landmarks for time. Because landmarks can be learned, geoscience programs might consider coordinated planning of key landmarks to be introduced at different stages of their academic programs.