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Gravity accelerates (or decelerates, depending on your point-of-view) time like ambulance's siren changes pitch (to your point-of-view) based on whether it's approaching or departing

In effect, gravity's time-dilation effect is functionally identical to the Doppler effect on sound

Time "travels" at a constant speed from its own point-of-view

Just like the Ambulance siren sounds the same, regardless of how fast the ambulance is moving, if you're inside the ambulance (ie, you share the point-of-view/frame-of-reference)

But from another point-of-view/frame-of-reference, it can appear to be moving slower or faster

This is the crux of general relativity with regards to the space-time continuum: it all depends on your frame-of-reference