The teaching of historical time is an important aspect in elementary school curricula. This study focuses on the effects of a curriculum intervention with "Timewise," a teaching approach developed to improve students' understanding of historical time using timelines as a basis with which students can develop their understanding of historical phenomena and periods. The study, in which 16 teachers from grade 2 (ages 7-8) and from grade 5 (ages 10-11) participated, represents the first curriculum intervention on the understanding of historical time in elementary schools in the Netherlands. The effects were measured in a quasi-experimental pre-/post-test design. Mixed model linear analyses showed that for both grade 2 and grade 5, students in the experimental condition (N = 396) scored significantly higher on the post-test than students in the control condition (N = 392), with a medium effect size. Implications for the teaching of historical time in elementary school and in teacher training are discussed.