This study uses a one-group pre-test and post-test design to describe the mental models of geologic time held by a group of Spanish pre-service primary teachers, before and after an instructional intervention that introduced the work with different timescales. The questionnaire was given before instruction and five weeks after it. It consists of a timeline where seven key events were to be sequenced, and two questions, one open-ended and one closed, related to the absolute ages of the events and their duration. The mental model that was repeated the most before and after instruction contemplates the planet and life as something that has evolved over time but is still quite far from the scientific model. The results also show a significant evolution of mental models towards the scientific model, although students still evidence certain difficulties when working with scales and absolute ages, tending to underestimate the spans of time involved. Furthermore, students exhibited certain difficulties when working with the ages of the most recent events that involve shorter time spans.