Objects have measures of length, width, and height. Objects also move, that is, travel and so change their position in one or more of the directions of length, width, and height. The relation of the position before and after movement is measured by the difference of length, width, and height, and these differences are called distances or durations.
Linear dimensions are measured by the circumference of a measuring wheel (or surveyor’s wheel) as it moves in a straight line along or parallel to the object being measured, beginning and ending with the extremities of the object. The duration of movement is measured by the angle swept out by measuring wheel (or the equivalent) as it moves in a straight line along or parallel to the object being measured, beginning and ending with the extremities of the object.
The speed of travel depends on the mode of travel (e.g., the type of propulsion, the medium, the vehicle, etc.). Each mode of travel likely has a maximum possible speed and a typical speed relative to a that mode of travel and perhaps other distinctions.
Thus far we have spoken about measurement and said nothing about space or time. That is because the measurement of distances and durations does not depend on a concept of space or time or space-time.
As we place objects into a conceptual realm, we need concepts of space and time as the conceptual context for linear and angular measures. This allows measures to be related to one another and inductive inferences to be made.
So linear measures are put into space and angular measures are put into time. Space and time may be conceived to extend indefinitely or to have a definite beginning and/or ending. Space and time may be given an origin to relate the position of all objects.
All of these concepts are conventions. So space and time and space-time are conventions. But the measurement of distance and duration are not conventions.