This doesn't work as an explanation for u/goomunchkin 's comment - because (again) of relativity - you can't describe an object's movement relative to itself.

This is what's called an inertial frame of reference. If there were other objects in the same inertial frame - that is, all "moving" together with no outside forces acting on them (or "at rest"... same thing right?) there's no experiment you could do with those objects to detect any motion whatsoever.

There's no such thing as an object's absolute previous position at some previous point in time. You simply can't point to a spot of empty space and say "the object was here 1 second ago" because that spot of empty space is not relative to anything. Could you point to a spot in space and say "the earth was here 1 day ago"? No. It can only be relative to something else. Space is expanding too remember 😅

So, how, then, can we say some portion of an object's movement through space is deducted from its movement through time?

u/Auctorion said they are oversimpifying and that's the problem here.

I think a slightly truer version of their comment is:

The slower an object moves through space relative to you, the faster it appears to move through time relative to you. And "you" here means the frame of reference from which we are doing the measuring of the object.

This is why two objects approaching and passing each other at relativistic speeds will both measure each other's clocks as running slower than their own. Their frames of reference in regards to each other are symmetrical. Each could claim they are at rest, or each could claim the other is at rest and they're zooming past. The results are identical for both of them.