This is the first, indisputable effect of what occured (whether it was calculated, well calculated, or not), precisely on September 11, not far from here: we repeat this, we must repeat it, and it is all the more necessary to repeat it insofar as we do not really know what is being named in this way, as of to exorcise two times at one go: on the one hand, to conjure away, as if by magic, the “thing” itself, the fear ot the terror it inspires (for repetition always protects by neutralizing, deadening, distancing a traumatism, and this is true for the repetition of the televised images we will speak of later), and, on the other hand, to deny, as close as possible to this act of language and this enunciation, our powerlessness to name in an appropriate fashion, to characterize, to think the thing in question, to get beyond the mere deictic of the date: something terrible took place on September 11, and in the end we don’t know what. For however outraged we might be at the violence, however much we might genuinely deplore―as I do, along with everyone else―the number of dead, no one will really be conviced that this is, in the end, what it’s all about.

― DERRIDA, Jacques ([2001]2003). «Autoimmunity: Real and Symbolic Suicides» in BORRADORI, Giovanna (2003). Philosophy in a Time of Terror, Chicago: The University of Chicago Press, p. 87
Read more via Google books preview. In the page just before this one (86) Derrida is commenting our collective effort to mark the event «in the shared archive of a universal calendar». Compare with Stiegler’s comment on the necessity of a shared calendar for the existence of a collective «us».