Alternative Time Systems

Or using a system of time not correlated to the rotation of the planet. (It is convenient though :( )

Why can't we use a sensible (moving forwards at a constant pace) time as a default for everyone, and have those rare astronomical systems use some special time options?

Case-in-point, you are mistaken. The duration of a day changes due to many things, both logically and also physically due to the nature of Earth. Also just because you can call a second a second doesn't mean that is helpful making datetime software usable or easy on a different planet.

As a kid, I used to wonder why time was never "corrected" into an easy-to-calculate system. Our calendars have months of varying length and nonsensical names (SEPTember, OCTober, NOVember and DECember should be they 7th, 8th, 9th and 10th months, but that didn't stop someone from shoving new months behind them to honour emporers).While 60, 60, 24, 7, 12, 365 don't make much sense (and might be improved on) the problem is that time and date are based on the planet's or

It would be impressive if days and seconds weren't defined arbitrarily.

Yes obviously we aren't sitting here measuring the earth's rotation to set our clocks. The point is the meaning of the number remains the same. Seconds, minutes and hours have meaning, and the meaning of those measured amounts of times is directly related to some fraction of the rotation of the planet we are currently sitting on. The point was moving this system of time-keeping to some other planet would discard much of its relevancy to our every-day life, which matters a lot.

Metric time and dates notably fail to work with the observed motion of the Solar system. The time metrics we use are determined by the rhythms of the Earth, Moon, and Sun. Metric time offers no benefit. (And for the record, Napoleon tried to decree decimal dates and times, but even the French weren't having any of that crap...)

Seconds since epoch is good enough for almost all purposes until you're moving at ~10% of the speed of light; they'll be fine for computer timestamps as long as acceleration and deceleration are gradual. So I don't think we'll need any new universal units of time, only new local units of time, and I'm not even sure about that. For mars, just extending the day until 24:40 = 00:00 seems reasonable enough. For Saturn's moons the sun is faint enough that keeping "earth time" is probably reasonable

Yeah, it does take you into a "what does time mean" rabbit hole. The sun is highly consistent, but that's not the same as convenient - we prefer to divide time into equal units, and leave the rotations of the earth to the International Earth Rotation Service.

No because its a humane rather than scientific system. Day lengths in a scientific time vary, but in this system the hours are defined to be 10ths of a solar day so they vary as the day varies. Scientific measurement should be done with a different system. I'd suggest something based on the speed of light.