Dasein's now-when.

In other words: Time is not something present out-there. It is not something that can be empirically intuited. But that means that time in itself is not determinable. I cannot determine any “now” in an absolute way by way of the pure pre-view of the whole of time, because any determinate “now” always already bespeaks a “now, when . . .” Every now-determination is essentially relative to some present thing; and only to the degree that this present thing (with regard to which time is determinable at all) can be fixed, is a determination of time possible.

By the way, Einstein arrived at this same framework for determining time by pursuing some quite specific, concrete problems in physics. The principle of the theory of relativity—that all time is the time of a certain place—is a principle that is grounded in the very essence of time, insofar as what is present in the sense of being present in nature can be determined only place-wise—i.e., only in terms of a place and relative to a place. There is no absolute perception of time. In a certain sense, as regards something present in nature, I can never simply and directly fix its “now” as given absolutely. Instead, the now is always a “now, when . . .”

P. 290