Krzysztof Lorek, Jorma Louko, Andrzej Dragan: Ideal clocks—a convenient fiction. Classical and Quantum Gravity 32,17 (2015) 175003 sounds interesting. According to Einstein, time passes at rates that depend on the observer, but I (at rest) could compute your rate by knowing your velocity. However -- might the rate of passage of time depend not only on velocity, but also on acceleration? Well, the "Unruh effect" says an accelerating observer will perceive more thermal noise. And this noise will cause your velocity to fluctuate as thermal background photons hit you and push you around. That in turn will alter your perception of time. So, yes. And actually, just sitting on Earth I am accelerating to counteract gravity all the time (in the general relativistic view) and am indeed experiencing a slightly different rate of time versus some observer far from Earth -- although for a different reason than I just said, this is because of the non-flatness of spacetime. The Unruhish effect just mentioned goes beyond merely general relativity. It should make quantum gravity nastier.