When you life in space, seasons do not have any meaning, and the periodicities of a year become arbitrary. You only want to measure time *intervals*, not cycles. So while the Martians have their calendar (since they actually do have to think about seasons and even the diurnal rhythm) it makes more sense to assume everybody are using old Earth years as the "year". Except of course Muslims, who might still be using a lunar calendar. And the fiscal calendar used by hypercorps. And habitats measuring time in periods ab urbe condita... Of course, the real measure of time is the second (unless you want to be hardcore and use Planck time multiples). ("I'm 1.215 gigaseconds old", "Just wait a hectosecond", "Cook for 870 seconds"). Purists like to point out whether it is seconds proper time (as measured by a co-moving observer) or seconds relative to a particular reference frame ("The message will arrive at Mars in 3,478 seconds, relative to our reference frame") Thank heavens for muses that just translate discreetly, so you can get everything in your own preferred format. ("The ship will arrive the 5th of Sektober in the Year of the Preening Gorilla") But absolute-time wise, I would say it is 10 Earth years since the Fall. 20 would give far too much time for stabilization. I like EP being at the point where life has returned to a semblance of stability rather than acute global crisis, yet has not settled down in any way.

Tree rings are determined by weather. So the rings will denote how many summer-winter cycles there has been - you could easily tweak them to fit whatever frequency you like.