So Planescape loved its taxonomies and endless demystifying, but they never mentioned time dilation. When it is 9am in Sigil, what time is it in Dis? Two twins left home, one twin moved to Arborea and another to Mechanus, when they meet again many years later will they be of equivalent age?
To help think about it, we need to decide on a measuring stick. Using the old rubber sheet analogy is difficult, since the planes don't have our universe of space and stars rolling around on it. No good thinking like that then. What about vortex theory? Spacetime enters from the centre of all solid matter, diffusing from an area of higher concentration to an area of lower concentration. The point of highest concentration also coincides with a zone of lowest entropy, the past. All energy then proceeds to a zone of high entropy, the future. The flow is time.
More suitably mysterious and mystical I think. Entropy manifests as the ragged edge of nothing, nibbling at existence, watched over by the horizon knights in their roaming fortresses. To the knights time moves quickly while they watch the nothing, circling the planar sink. In the city at the heart of creation time hardly moves at all, giving its citizen a unique vantage and the leisure of philosophising and discussing the minutiae of life, the universe and everything. When the discussions come to talking about the outside world one can pass through a portal, spend years investigating first hand, and be back in time for next week's meeting of minds.
You couldn't create robust rulings for time without explaining the universe to death, but knowing that The City is almost static and the outer edges are roiling rapids allows an amount of sense. In practise consider the time of another plane to be completely disconnected to the one you stand on and with only the centre holding. While the party's activity is in one place they can rely on their sense of time and space, if they leave time moves at the speed of drama.