Sir,

Time (in this context) is a measurement framework, calibrated in standard units (e.g. hours). It calibrates change (hence ’rate’) So, yes, Time is constant. It is also reference-frame specific.

Space too (in this context) is also a measurement framework, calibrated in standard units (eg miles). It calibrates relative position.

Spacetime hence calibrates changing position ie. Constant Motion (constant because all calibrations are reference-frame specific). It is calibrated in miles per hour.

Spacetime-time calibrates constant acceleration eg miles per hour per hour.

Spacetime-time-time calibrates constantly changing acceleration. Miles per hour per hour per hour.

And so on.

That makes spacetime complex.

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