Modern Philosophy of Science has not yet recognized the significance of physical invariance principles for science and daily life. In this paper we investigate as a simple example, how time independence or time translational invariance of natural laws determines the time scale. We start with informal definitions of the invariance of concepts and laws. We then ask if time independence is an essential feature of natural laws or if time dependent laws are plausible without loss of predictive relevance. In section (...) 3 we investigate the question wether time independence of natural laws is a matter of convention. In sections 5-7 the determination of the time scale by time translational invariance is studied. In section 8 we study the conditions under which this determination is unique. (shrink)