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I gave it quite some thought, but like you, the only alternative that I could see was using the dates. To address the very real problem of stories taking place concurrently, I do think that the simplest answer is a range. It would be quite easy to put something like the following at the bottom of each cover/spine:
2683/11/27 22:30
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2683/2/14 14:25
or just:
2683/11/27
-
2683/2/14
if you decide that hours are redundant, as it is visually preferable. This has a number of advantages, in that it is, as you said, infinitely expandable, and you will never have awkward problems about introducing a new era or series. It also becomes readily apparent which order stories should likely be read in, even if stories are overlapping or contained within another's timeline.
Finally, a small detail; you should put a complete (to-date) timeline of all novels/novellas/anything in the first few pages. Despite never being fully comprehensive, it is an invaluable resource, and one that I greatly appreciated having in other series.
Well, that's my two cents.

