This chapter begins by outlining Heidegger’s project of identifying the timeliness (Zeitlichkeit) of human existence as what is ontologically distinctive about it. The chapter also recounts how, in the context of establishing that distinctiveness, Heidegger demonstrates that timeliness to be the “original time,” that is, the origin of so-called “world-time” (Weltzeit) (the time of the workworld) and, via use of the clock, the origin of the purely serial time attributed to things on hand in nature. In the wake of this exposition and after flagging criticisms of Heidegger’s undertaking, the chapter examines Ernst Tugendhat’s influential criticism that Heidegger’s putative demonstration is invalid since it has recourse to serial time (“time in the normal sense”) and, hence, is viciously circular. The chapter ends with a sustained rebuttal of Tugendhat’s criticism.