The main point, it seems, is (to quote Michael Inwood) 'time and space are not for Heidegger co-ordinate,' because he believes that our special mode of 'hey-here-we-are!' possesses a spatiality 'based on or "embraced" by temporality.' [p.222]. This sounds like Heidegger believes that time has a kind of priority over space, and that he disbelieves in spacetime (which is to say, as far as we can see, that he's on the wrong track); but in Being and Time it doesn't seem quite so simple as that.
'Dasein's specific spatiality must be grounded in temporality. On the other hand, the demonstration that this spatiality is existentially possible only through temporality, cannot aim either at deducing space from time or at dissolving it into pure time. If Dasein's spatiality is "embraced" by temporality in the sense of being existentially founded upon it, then this connection between them is also different from the priority of time over space in Kant's sense.' [p.418]
So, let me see if I understand. Time is prior to space, but not in the sense that Kant thought (so, for example: it is not that there was time and then, bang, along came space; a time into which space came). We can, let us say, move about in space in ways we can't in time: I can go into any corner of this room, but I can only go one way in time. At the same time, it is a simple cognitive mistake to spatliaze time: to think of it as a portion of space that must be travelled over, or talk of the 'river of time' or 'the city at the end of time' or 'the terminal beach'. Then there's the notion that time is somehow produced or generated: in German zeitigen, 'to make ripen, to bring to maturity, to bring about, produce'. Hence Heidegger talks about extemporising, not in the usual English sense but in the sense (like the standing-outside of ec-stasy) of the throwing out timelinessess: 'temporality "is" not an entity at all. It is not, but it temporalizes itself ... indeed, it temporalizes possible ways of itself.' [p.377]. I look at that and try to see whether it is describing what it seems to be describing, the many-worlds hypothesis, various possible futures (say) branching or webbing off from any given moment. Or, no: Heidegger won't have us isolating 'moments', not even (I suspect) as an exercise in conceptual modelling or calculus: 'the nadir of inauthentic temporality is "time as a sequence of nows" or instants, time conceived apart from Dasein's activities and purposes.'
I can't remove from my head the deliberate inversion of what Heidegger means: not time-space (Zeitraum) but time-light, or time-gravity. As with light, which is generated (ex-illuminated) by stars; or with gravity, the true grain of space, which is in-drawn by matter, so with time: at the heart of the cosmos as a whole, a vast Time Sun temporalising with a steady billennial flow time, time.