Re: Metric time
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Well the sun rotates about its axis once every 25 days (at its equator) so I think that would be a good starting point. Obviously if we inhabit multiple planets then basing our calendar on the orbit of one will make the seasons of the other asynchronous. It would be best to clock the rotations of the sun (since the whole solar system can agree on this*). Also 25 days is pretty close to a month's length anyways, which makes it an intuitive amount of time. The year could be a planetary abstraction about time, where we use 'solar months' to specify exactly when something happened invariant of location (i.e. I was born 24,017.55 solar months BCE2).
* staying synchronously timed in the inhabited solar system will be quite a pain in the ass though because of the effects of relativity. Still measuring time from the behavior of the sun would allow travelers to calculate the discrepancy.
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Problem is that a sun's rotation doesn't have much meaning for us. You can watch sunspots to detect rotation, but that's about it, so it's really just an arbitrary measure. And you're right about relativity issues. I imagine using an atomic clock based on earth (days/years like now) is natural -- at least they have some meaning as earth is our home planet and we've used this standard for thousands of years, so seems as good an arbitrary reference as any.
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