The Thesis of Being and Time Part 1: Der Zeitbegriff als Vorstellung

Aporia 1: A concept of time is a representation of time. A representation sets something before us (vor-stellung) in such a way that this thing is made present again.  In order to represent time, we must make present again not only the present, but the past and the future. We must therefore make present that which can never be found in the present. 

Aporia 2: In the representation of time, not only must we present again what is not  --nor could ever be --present. We must also be able to perform this presentation precisely from out of the present. In other words, there is the horizon of time to be made present, and there is the horizon of time in which such a presentation is to take place. The  horizon of time to be represented (H2) must be re-presented in the present horizon of time (H1). 

Aporia 3: The horizon of time (H1) in which this representation of the horizon of time (H2)  takes place, although it is the present time of the act of representation, is not included in the concept being represented, but serves as its implicit condition. The present horizon of time (H1) is either no longer or not yet represented in the concept of time. But, as was stated in Aporia 1 above, it belongs uniquely to the concept of time as its distinguishing feature and task  to make present what is no longer (the past) and not yet (the future).

Conclusion: The result is a familiar paradoxical turn of phrase. Most obtusely and oracularly, it claims: the concept of time is the time of the concept. Or in other words, to attempt to conceive of time is to attempt to conceive of the condition of that conception. Time is implicit in its own conception.