Yeah, going back to my quip regarding geologists, that is something that could be done as well in a lore sense: having named eons and eras and then referring to years within those eras rather than having all dates relative to one single fixed point.
That probably doesn’t help with sorting though.
If sorting is important, you might find you need to run dual metadata fields to get what you want. Have a second field set up that uses a purely relative calendar. Such as,
1mil BC = 0
900k BC = 100k
50 BC = 950k
1AD = 1m1
100 AD = 1m100
Etc
And then sort on that field.
You can then have a second metadata field that contains the date as you’d write it in your story. You’ll need to manually update both fields if you change a date, of course (which could cause errors to creep in if you forget). Also, you’ll need to give yourself plenty of headroom on the reference system… it’ll be a pain in the backside if you have to go and correct every entry because you used 1milBC as your reference point but then have to change it because something really cool needs to happen in 1mil5 BC! 