Despite growing advocacy of interdisciplinary approaches to science, technology, engineering and mathematics (STEM), there are persistent concerns about the practical and principled epistemic bases on which this can be justified as a mainstream curricular practice. A major issue concerns the nature of interrelations between the STEM disciplines in interdisciplinary work. In the paper, we argue that the spatial metaphor prevalent in discussions of interdisciplinarity, and boundary crossing, while useful, has limitations in capturing the challenges and possibilities for interdisciplinary STEM in schools and in some respects misconstrues the cultural and historical nature of disciplinary epistemic practices. We argue, as a contribution to a theory of interdisciplinary curricular practice, that key issues for such practice can be productively seen through a temporal metaphor and that a temporal model can usefully inform schools and teachers in framing and supporting student learning in interdisciplinary settings. To illustrate this, we use indicative cases from two major Australian STEM initiatives to examine temporal relations between mathematics, science and technology, at the micro-, meso- and macro-levels, and the implications of this for disciplinary integrity. Finally, we argue that contemporary interdisciplinary advocacy amounts to a challenge to renew epistemic traditions in the STEM subjects.