For this reason most SF writers fudge the issue by referring to "Standard Cycles" or "Solar Days" or "Median Orbits". They can always make a comment when talking to other alien characters. Perhaps characters could just work off of major events.
The nomenclature is alien enough to be exotic whilst clear enough to imply that time passes at the same rate. I think the "Stardate" thing was just a device to make the voyage of the USS Enterprise appear to be timeless and epic. I would always aim to put the calendar according to the POV character. Is it important for you to be specific in terms of how many years have passed between events?
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Currently, I'm using Earth dates, something like: July 28 (Earth date), aliens attack Pluto, August 20 move to Mars etc.
Dates and times are internationally standardized in ISO 8601.
The ISO 8601 dates use the Gregorian calendar and 24-hour time, i.e.
But people on Pluto wouldn't use the Earth calendar; they would use their own system.
Like: When we give alien dialog in a book, we routinely give it in English, even though it would seem somewhat unlikely that aliens from another star system would speak English. So translating dates also would not be implausible.Especially if they're in regular contact with Earth people.Sure, the Gregorian calendar does not match the orbital period of Mars or Pluto, but the convenience of using the same calendar on all the planets in the solar system would likely outweigh any advantage of having the calendar match the orbital period.People working in locations where the days are unusually long or short (or practically non-existent e.g.the South Pole) experience bizarre physiological and psychological effects.We have the aliens give distances in light-years, even though they would surely not use this particular unit. Where it WOULD get implausible is: (a) Anything that assumes that dates "line up".