The ability of hydrological models to perform in climatic conditions different from those encountered in calibration is crucial to ensure a reliable assessment of the impact of climate change on river regimes and water availability. However, most evaluation studies based on the differential split-sample test (DSST) endorsed the consensus that rainfall-runoff models lack climatic robustness. Models applied under climatologically different conditions typically exhibit substantial errors in streamflow volumes. In this technical note, we propose a new performance metric to evaluate model robustness without applying the DSST, and it can be performed with a single hydrological model calibration. The proxy for model robustness (PMR) is based on the systematic computation of model error on sliding sub-periods of the whole streamflow time series. We demonstrate that the PMR metric shows patterns similar to those obtained with the DSST for a conceptual model on a set of 377 French catchments. An analysis of the sensitivity to the length of the sub-periods shows that this length influences the values of the PMR and its equivalency with DSST biases. We recommend a range of a few years for the choice of sub-period lengths, although this should be context dependent. Our work makes it possible to evaluate the temporal transferability of any hydrological model, including uncalibrated models, at a very low computational cost.