Comparison group selection is paramount for health policy evaluations, where randomization is seldom practicable. Rolling enrolment is common in these evaluations, introducing challenges for comparison group selection and inference. We propose a novel framework, GroupMatch, for comparison group selection under rolling enrolment, founded on the notion of time agnosticism: two subjects with similar outcome trajectories but different enrolment periods may be more prognostically similar and produce better inference if matched, than two subjects with the same enrolment period but different pre‐enrolment trajectories. We articulate the conceptual advantages of this framework and demonstrate its efficacy in a simulation study and in an application to a study of the effect of falls in Medicare Advantage patients.