which can obtain only in a synthesis in accordance with rules. (B 263)108
Every determination of nature, as a synthesis, is grounded in the original synthesis of apperception. But insofar as it is a determination of appearances that are encountered in time, every determination of nature is a determination of the being-in-time of nature,  and thus a determination of time. If it is to determine objects, the synthesis is bound to a view of time in general. The rules of the synthesis are co-determined in terms of time. A synthesis is essentially a determination of time; and, as a synthesis of the scientific knowledge of nature, it is a determination of the objective being-in-time.
But what does it mean to “determine time”? We first ask: What does an empirical determination of time require? And secondly: What does an objective-scientific determination of time require? We shall take the explanation only as far as is necessary to interpret the First Analogy and the schematism.
Time (that is, the time wherein the data appear) is to be determined empirically. We start with the way in which time is first given in Kant’s sense, and then analyze these modes of the givenness of time. We are not saying that this mode of time-givenness is the primary mode within natural experience. It is primary only on the basis of Kant’s starting point. For Kant, what is first given is the manifold of presentations in inner sense. If I behave purely passively—i.e., if I just let that manifold be given to me—then one can see that the “presentations” are given in [the form of] pure change: There is the beginning of something and the end of it: this, that, and the next thing. What is present only for a while changes, and in its place comes something else—now this, now that, a pure perception of what is present. There is only change (never a one-after-the-other succession) as long as I abandon myself simply and directly to whatever is present—in which case I let myself be simply taken prisoner by what is present, without following it toward its disappearance or looking at what might come after it. I cannot determine this pure change temporally; or more exactly: as given over to this pure change, I do not comport myself in a time-determining way.
I do that only when I consider the time-character of what is currently present and changing, and say “now.” But even when I do that, I could just say “now, now.” Only when I take hold of the time-character of the  self-repeating changing things, only then is there ever a now. But if I also try somehow to determine this “now” and
108.[CPR, p. 320.]