[flopunctro]

AFAIK, there is no canonical time in relativity. There is the "local time", which is what your wristwatch shows, and the "time in place X", where X can be Earth, Mars, or some spaceship. These times might not be in sync, and they might even be distorted. Because, if place X is moving relative to you at a significant percentage of lightspeed, their seconds will be longer. Also, the further X is, the blurrier the concept of simultaneity becomes. Which makes the question "what time is it _now_ in place X" moot :)

[numpad0]

If relativity don’t allow time travel doesn’t it make time canonical? A clock on Earth from telescope might look jumpy but only in forward direction after compensating for distance I think

[flopunctro]

Looking at a clock from Earth through a telescope isn't as straightforward as you'd think. The image you see of that clock is actually light (photons) emitted from Earth, which will take a while to reach you - like, 1 year, if you're 1 lightyear away. During that year, Earth has moved on, maybe blew itself apart. But you can't even tell, because information can't reach you faster than light :) So you can only see Earth's past, not Earth's "now". The further you are, the more "now" loses meaning.

[numpad0]

Yeah but can it ever go back, other than by backing off faster than the speed of light? If it can’t then it’s at least monotonic, even if it’s not rate constant

[dodobirdlord]

All observers observer all clocks to advance monotonically forward in time, regardless of their location or relative speed, unless the clock is moving at the speed of light. But the rate that each clock is observed to advance at depends on things like relative speed, acceleration, and the curvature of spacetime.

[JdeBP]

... unless it's the system clock in Linux, with the hardware RTC set to be interpreted as local time, and one is watching a system boot on a machine that is set up as east of the Prime Meridian (i.e. the hardware RTC is ahead of UTC). (-:

* https://unix.stackexchange.com/a/600490/5132

[dodobirdlord]

Given that Unix time specifically doesn’t handle leap seconds, a system clock is not a clock by the conventional definition.