One of the fundamental parameters in quantification of sputter depth profiles is the sputtered depth. Usually, the sputtered depth is not known from the measured profile, because only the sputtering time is recorded. Only if the instantaneous sputtering rate (=sputtered depth per unit of time) is known, the depth scale can be derived from the time scale. To determine the average sputtering rate, a certain sputtered depth has to be correlated with a measured sputtering time. In general, the sputtered depth has to be determined by additional measurements that is the topic of a recently published ISO Technical Report (ISO/TR15969). In this work, we focus on the correction of the time scale in terms of a true depth scale in case of preferential sputtering of one component of a binary system with concentration variation, as shown for the example of a Si/Ta multilayer. Using a modified version of the MRI model, it is shown how the correct the time/depth relation for quantification of sputter depth profiles is obtained.