Special relativity joins spatial and temporal coordinates in spacetime and makes distant simultaneity frame-dependent. General relativity later describes gravity through spacetime geometry. These are related theories, not one claim that mass simply 'bends time.'

A moving clock accumulates less proper time between suitable reunion events than a clock following another path; gravitational potential also changes clock rates. Time does not slow down in order to compensate for light. The Lorentz geometry of spacetime produces both invariant light speed and time dilation.

Light cones organise which events can influence which others. A signal or local influence cannot outrun light in vacuum; observers agree on the causal order of events connected within those cones even when they disagree about the order of spacelike-separated events.

Calling c the 'speed of causality' is useful shorthand. Calling it reality's maximum update rate imports a computer mechanism physics does not establish. Events outside one event's light cone are causally separated from that event then; some may enter a later observer's causal past or future as time passes. 'Outside' does not mean eternally unreachable in every case.

Nearby clocks can be synchronised by a chosen procedure, so ordinary life supports a useful shared 'now.' Special relativity says observers in relative motion need not extend that simultaneity across distance in the same way. The difference is relational, not a private time owned by each person.

GPS makes the geometry operational. Motion would make satellite clocks lose about seven microseconds per day relative to ground clocks, while weaker gravity makes them gain about 45; the net is about 38 microseconds per day and must be corrected. Relativity is not merely a thought experiment. It helps place the blue dot in your pocket.