After recalling current performances of time and frequency standards and their use to establish time scales, their limitations in astronomical observations and studies are considered. For expressing the value of almost all quantities, the accuracy of the frequency standards is sufficient. However, an improved stability could find applications in observation techniques and the departure of realized time scales with respect to their theoretical counterparts is not negligible for some studies. The paper recalls the basic rule of quantity calculus and advocates its use in spite of its inadequacy to express difference of readings of time scales. In general, adherence to the metrological practice and the use of the SI may facilitate our work and its understanding by the non-astronomers.