Yes it is, outside of our universe. Just because that's the way it works living in our three dimensions doesn't mean that's the way it works elsewhere. Since you can have no knowledge of what happens outside of our universe how could it not be an assumption
Time is the rates of change within the physical dimensions of height, width and depth. This applies to two dimensions or more. Time is inseparable from space, ie, timespace. Check out general relativity.
